diff --git a/src/js/_enqueues/admin/post.js b/src/js/_enqueues/admin/post.js
index d50fe6007d33b..9a5bbd7309acd 100644
--- a/src/js/_enqueues/admin/post.js
+++ b/src/js/_enqueues/admin/post.js
@@ -316,7 +316,46 @@ jQuery( function($) {
 		copyAttachmentURLSuccessTimeout,
 		__ = wp.i18n.__, _x = wp.i18n._x;
 
+	/**
+	 * Show a warning in the classic editor when the post content includes
+	 * serialized block comments, which may not be visible in the Visual tab.
+	 */
+	function maybeShowBlockMarkupWarning() {
+		var hasBlockMarkup, $warning, $editorWrapper;
+
+		if ( ! $textarea.length ) {
+			return;
+		}
+
+		hasBlockMarkup = /<!--\s*wp:[^>]*-->/.test( $textarea.val() );
+		if ( ! hasBlockMarkup ) {
+			return;
+		}
+
+		$warning = $( '#classic-editor-block-markup-notice' );
+		if ( $warning.length ) {
+			return;
+		}
+
+		$warning = $( '<div>', {
+			id: 'classic-editor-block-markup-notice',
+			'class': 'notice notice-warning inline'
+		} ).append(
+			$( '<p>' ).text(
+				__(
+					'This content includes Gutenberg block markup that may be visible in the Code editor but not in the Visual editor. The page may not be empty. Review the content carefully before overwriting or deleting it.'
+				)
+			)
+		);
+
+		$editorWrapper = $( '#wp-content-wrap' );
+		if ( $editorWrapper.length ) {
+			$editorWrapper.before( $warning );
+		}
+	}
+
 	postboxes.add_postbox_toggles(pagenow);
+	maybeShowBlockMarkupWarning();
 
 	/*
 	 * Clear the window name. Otherwise if this is a former preview window where the user navigated to edit another post,
diff --git a/src/wp-admin/includes/admin-filters.php b/src/wp-admin/includes/admin-filters.php
index 5337cc02c88c9..78bad24c1571f 100644
--- a/src/wp-admin/includes/admin-filters.php
+++ b/src/wp-admin/includes/admin-filters.php
@@ -45,6 +45,7 @@
 // Misc hooks.
 add_action( 'admin_init', 'wp_admin_headers' );
 add_action( 'admin_init', 'send_frame_options_header', 10, 0 );
+add_action( 'admin_init', 'wp_maybe_register_performance_optimization_settings' );
 add_action( 'admin_head', 'wp_admin_canonical_url' );
 add_action( 'admin_head', 'wp_site_icon' );
 add_action( 'admin_head', 'wp_admin_viewport_meta' );
diff --git a/src/wp-admin/includes/ajax-actions.php b/src/wp-admin/includes/ajax-actions.php
index 2af08fba70af9..c4d8bba992bac 100644
--- a/src/wp-admin/includes/ajax-actions.php
+++ b/src/wp-admin/includes/ajax-actions.php
@@ -2321,7 +2321,7 @@ function wp_ajax_widgets_order() {
 	unset( $_POST['savewidgets'], $_POST['action'] );
 
 	// Save widgets order for all sidebars.
-	if ( is_array( $_POST['sidebars'] ) ) {
+	if ( isset( $_POST['sidebars'] ) && is_array( $_POST['sidebars'] ) ) {
 		$sidebars = array();
 
 		foreach ( wp_unslash( $_POST['sidebars'] ) as $key => $val ) {
diff --git a/src/wp-admin/menu.php b/src/wp-admin/menu.php
index 57d94c75e26f2..a7745676bd8d1 100644
--- a/src/wp-admin/menu.php
+++ b/src/wp-admin/menu.php
@@ -409,6 +409,7 @@ function _add_plugin_file_editor_to_tools() {
 	$submenu['options-general.php'][20] = array( __( 'Reading' ), 'manage_options', 'options-reading.php' );
 	$submenu['options-general.php'][25] = array( __( 'Discussion' ), 'manage_options', 'options-discussion.php' );
 	$submenu['options-general.php'][30] = array( __( 'Media' ), 'manage_options', 'options-media.php' );
+	$submenu['options-general.php'][35] = array( __( 'Performance' ), 'manage_options', 'options-performance.php' );
 	$submenu['options-general.php'][40] = array( __( 'Permalinks' ), 'manage_options', 'options-permalink.php' );
 	$submenu['options-general.php'][45] = array( __( 'Privacy' ), 'manage_privacy_options', 'options-privacy.php' );
 
diff --git a/src/wp-admin/options-performance.php b/src/wp-admin/options-performance.php
new file mode 100644
index 0000000000000..c47d18eaa1c30
--- /dev/null
+++ b/src/wp-admin/options-performance.php
@@ -0,0 +1,145 @@
+<?php
+/**
+ * Performance settings administration panel.
+ *
+ * @package WordPress
+ * @subpackage Administration
+ */
+
+/** WordPress Administration Bootstrap */
+require_once __DIR__ . '/admin.php';
+
+if ( ! current_user_can( 'manage_options' ) ) {
+	wp_die( __( 'Sorry, you are not allowed to manage options for this site.' ) );
+}
+
+global $wpdb;
+
+$title       = __( 'Performance Settings' );
+$parent_file = 'options-general.php';
+
+wp_register_performance_optimization_settings();
+
+if ( isset( $_POST['performance_action'] ) ) {
+	check_admin_referer( 'performance-tools' );
+
+	if ( 'clear_cache' === $_POST['performance_action'] ) {
+		wp_delete_performance_cache();
+		add_settings_error( 'performance', 'performance_cache_cleared', __( 'Performance caches cleared.' ), 'success' );
+	} elseif ( 'database_cleanup' === $_POST['performance_action'] ) {
+		$deleted  = 0;
+		$deleted += (int) $wpdb->query( "DELETE FROM $wpdb->posts WHERE post_type = 'revision'" );
+		$deleted += (int) $wpdb->query( "DELETE FROM $wpdb->comments WHERE comment_approved = 'spam'" );
+		$deleted += (int) $wpdb->query( "DELETE FROM $wpdb->comments WHERE comment_approved = 'trash'" );
+		$deleted += (int) $wpdb->query( "DELETE FROM $wpdb->postmeta WHERE meta_key = '_edit_lock' OR meta_key = '_edit_last'" );
+		wp_clean_performance_page_cache();
+
+		/* translators: %d: Number of database rows deleted. */
+		add_settings_error( 'performance', 'performance_database_cleaned', sprintf( __( 'Database cleanup complete. %d rows were removed.' ), $deleted ), 'success' );
+	}
+}
+
+get_current_screen()->add_help_tab(
+	array(
+		'id'      => 'overview',
+		'title'   => __( 'Overview' ),
+		'content' => '<p>' . __( 'Performance settings provide native controls for front-end caching, asset minification, critical CSS generation, lazy loading, image optimization, and database cleanup.' ) . '</p>',
+	)
+);
+
+require_once ABSPATH . 'wp-admin/admin-header.php';
+
+$settings = wp_get_performance_optimization_settings();
+
+?>
+
+<div class="wrap">
+<h1><?php echo esc_html( $title ); ?></h1>
+
+<?php settings_errors( 'performance' ); ?>
+
+<form action="options.php" method="post">
+<?php settings_fields( 'performance' ); ?>
+
+<h2 class="title"><?php _e( 'Optimization features' ); ?></h2>
+<p><?php _e( 'Choose which native optimization features WordPress should apply.' ); ?></p>
+
+<table class="form-table" role="presentation">
+	<tr>
+		<th scope="row"><?php _e( 'Page caching' ); ?></th>
+		<td>
+			<label for="performance_optimization_page_cache">
+				<input name="performance_optimization[page_cache]" type="checkbox" id="performance_optimization_page_cache" value="1" <?php checked( $settings['page_cache'] ); ?> />
+				<?php _e( 'Cache anonymous front-end pages for faster repeat visits.' ); ?>
+			</label>
+		</td>
+	</tr>
+	<tr>
+		<th scope="row"><?php _e( 'CSS and JavaScript minification' ); ?></th>
+		<td>
+			<label for="performance_optimization_minify_assets">
+				<input name="performance_optimization[minify_assets]" type="checkbox" id="performance_optimization_minify_assets" value="1" <?php checked( $settings['minify_assets'] ); ?> />
+				<?php _e( 'Minify front-end HTML output and inline CSS and JavaScript.' ); ?>
+			</label>
+		</td>
+	</tr>
+	<tr>
+		<th scope="row"><?php _e( 'Critical CSS' ); ?></th>
+		<td>
+			<label for="performance_optimization_critical_css">
+				<input name="performance_optimization[critical_css]" type="checkbox" id="performance_optimization_critical_css" value="1" <?php checked( $settings['critical_css'] ); ?> />
+				<?php _e( 'Generate a compact critical CSS block from early inline styles.' ); ?>
+			</label>
+		</td>
+	</tr>
+	<tr>
+		<th scope="row"><?php _e( 'Lazy loading' ); ?></th>
+		<td>
+			<label for="performance_optimization_lazy_loading">
+				<input name="performance_optimization[lazy_loading]" type="checkbox" id="performance_optimization_lazy_loading" value="1" <?php checked( $settings['lazy_loading'] ); ?> />
+				<?php _e( 'Automatically lazy-load eligible images and iframes.' ); ?>
+			</label>
+		</td>
+	</tr>
+	<tr>
+		<th scope="row"><?php _e( 'Image optimization' ); ?></th>
+		<td>
+			<label for="performance_optimization_image_optimization">
+				<input name="performance_optimization[image_optimization]" type="checkbox" id="performance_optimization_image_optimization" value="1" <?php checked( $settings['image_optimization'] ); ?> />
+				<?php _e( 'Convert generated JPEG and PNG image sizes to AVIF or WebP when supported.' ); ?>
+			</label>
+		</td>
+	</tr>
+	<tr>
+		<th scope="row"><?php _e( 'Database cleanup tools' ); ?></th>
+		<td>
+			<label for="performance_optimization_database_cleanup">
+				<input name="performance_optimization[database_cleanup]" type="checkbox" id="performance_optimization_database_cleanup" value="1" <?php checked( $settings['database_cleanup'] ); ?> />
+				<?php _e( 'Show cleanup controls for revisions, spam, trash, and temporary editing metadata.' ); ?>
+			</label>
+		</td>
+	</tr>
+</table>
+
+<?php submit_button(); ?>
+</form>
+
+<hr />
+
+<h2 class="title"><?php _e( 'Maintenance tools' ); ?></h2>
+<form method="post">
+	<?php wp_nonce_field( 'performance-tools' ); ?>
+	<input type="hidden" name="performance_action" value="clear_cache" />
+	<?php submit_button( __( 'Clear performance caches' ), 'secondary', 'submit', false ); ?>
+</form>
+
+<?php if ( $settings['database_cleanup'] ) : ?>
+	<form method="post" style="margin-top: 1em;">
+		<?php wp_nonce_field( 'performance-tools' ); ?>
+		<input type="hidden" name="performance_action" value="database_cleanup" />
+		<?php submit_button( __( 'Clean up database' ), 'secondary', 'submit', false ); ?>
+	</form>
+<?php endif; ?>
+</div>
+
+<?php require_once ABSPATH . 'wp-admin/admin-footer.php'; ?>
diff --git a/src/wp-includes/default-filters.php b/src/wp-includes/default-filters.php
index b6295ddf19ed1..e38c4d59f74d6 100644
--- a/src/wp-includes/default-filters.php
+++ b/src/wp-includes/default-filters.php
@@ -363,6 +363,7 @@
 add_action( 'wp_footer', 'wp_print_footer_scripts', 20 );
 add_action( 'template_redirect', 'wp_shortlink_header', 11, 0 );
 add_action( 'wp_print_footer_scripts', '_wp_footer_scripts' );
+add_action( 'template_redirect', 'wp_start_performance_page_cache', 0 );
 add_action( 'init', '_register_core_block_patterns_and_categories' );
 add_action( 'init', 'check_theme_switched', 99 );
 add_action( 'init', array( 'WP_Block_Supports', 'init' ), 22 );
@@ -537,6 +538,7 @@
 add_action( 'init', 'rest_api_init' );
 add_action( 'rest_api_init', 'rest_api_default_filters', 10, 1 );
 add_action( 'rest_api_init', 'register_initial_settings', 10 );
+add_action( 'rest_api_init', 'wp_register_performance_optimization_settings', 10 );
 add_action( 'rest_api_init', 'create_initial_rest_routes', 99 );
 add_action( 'parse_request', 'rest_api_loaded' );
 
@@ -795,6 +797,18 @@
 add_action( 'before_delete_post', '_wp_before_delete_font_face', 10, 2 );
 add_action( 'init', '_wp_register_default_font_collections' );
 
+// Collaboration.
+add_action( 'admin_init', 'wp_collaboration_inject_setting' );
+
+// Performance optimization.
+add_action( 'save_post', 'wp_clean_performance_page_cache' );
+add_action( 'deleted_post', 'wp_clean_performance_page_cache' );
+add_action( 'clean_post_cache', 'wp_clean_performance_page_cache' );
+add_action( 'transition_comment_status', 'wp_clean_performance_page_cache' );
+add_filter( 'wp_get_loading_optimization_attributes', 'wp_performance_filter_loading_optimization_attributes', 10, 2 );
+add_filter( 'image_editor_output_format', 'wp_performance_filter_image_editor_output_format' );
+
+// Add ignoredHookedBlocks metadata attribute to the template and template part post types.
 // Add ignoredHookedBlocks metadata attribute to the template and template part post types.
 add_filter( 'rest_pre_insert_wp_template', 'inject_ignored_hooked_blocks_metadata_attributes' );
 add_filter( 'rest_pre_insert_wp_template_part', 'inject_ignored_hooked_blocks_metadata_attributes' );
diff --git a/src/wp-includes/performance.php b/src/wp-includes/performance.php
new file mode 100644
index 0000000000000..05c819e2c7a75
--- /dev/null
+++ b/src/wp-includes/performance.php
@@ -0,0 +1,352 @@
+<?php
+/**
+ * Performance optimization functions.
+ *
+ * @package WordPress
+ */
+
+function wp_get_performance_optimization_defaults() {
+	return array(
+		'page_cache'         => false,
+		'minify_assets'      => false,
+		'critical_css'       => false,
+		'lazy_loading'       => true,
+		'image_optimization' => false,
+		'database_cleanup'   => false,
+	);
+}
+
+function wp_sanitize_performance_optimization_settings( $settings ) {
+	$defaults = wp_get_performance_optimization_defaults();
+
+	if ( ! is_array( $settings ) ) {
+		$settings = array();
+	}
+
+	$sanitized = array();
+	foreach ( $defaults as $feature => $default ) {
+		$sanitized[ $feature ] = isset( $settings[ $feature ] ) ? wp_validate_boolean( $settings[ $feature ] ) : $default;
+	}
+
+	return $sanitized;
+}
+
+function wp_get_performance_optimization_settings() {
+	$settings = get_option( 'performance_optimization', array() );
+
+	return wp_parse_args(
+		wp_sanitize_performance_optimization_settings( $settings ),
+		wp_get_performance_optimization_defaults()
+	);
+}
+
+function wp_performance_optimization_enabled( $feature ) {
+	$settings = wp_get_performance_optimization_settings();
+
+	return ! empty( $settings[ $feature ] );
+}
+
+function wp_register_performance_optimization_settings() {
+	global $wp_registered_settings;
+
+	if ( isset( $wp_registered_settings['performance_optimization'] ) ) {
+		return;
+	}
+
+	register_setting(
+		'performance',
+		'performance_optimization',
+		array(
+			'type'              => 'object',
+			'description'       => __( 'Native performance optimization settings.' ),
+			'sanitize_callback' => 'wp_sanitize_performance_optimization_settings',
+			'default'           => wp_get_performance_optimization_defaults(),
+			'show_in_rest'      => array(
+				'schema' => array(
+					'type'                 => 'object',
+					'additionalProperties' => false,
+					'properties'           => array(
+						'page_cache'         => array(
+							'type'        => 'boolean',
+							'description' => __( 'Enable built-in page caching for anonymous front-end requests.' ),
+						),
+						'minify_assets'      => array(
+							'type'        => 'boolean',
+							'description' => __( 'Minify front-end HTML and inline CSS and JavaScript.' ),
+						),
+						'critical_css'       => array(
+							'type'        => 'boolean',
+							'description' => __( 'Generate a small cached critical CSS block from early inline styles.' ),
+						),
+						'lazy_loading'       => array(
+							'type'        => 'boolean',
+							'description' => __( 'Add lazy-loading attributes to eligible images and iframes.' ),
+						),
+						'image_optimization' => array(
+							'type'        => 'boolean',
+							'description' => __( 'Prefer modern image output formats when supported by the server.' ),
+						),
+						'database_cleanup'   => array(
+							'type'        => 'boolean',
+							'description' => __( 'Show database cleanup tools on the Performance settings screen.' ),
+						),
+					),
+				),
+			),
+		)
+	);
+}
+
+function wp_maybe_register_performance_optimization_settings() {
+	global $pagenow;
+
+	if ( wp_doing_ajax() ) {
+		return;
+	}
+
+	$is_performance_screen = 'options-performance.php' === $pagenow;
+	$is_options_screen     = 'options.php' === $pagenow;
+
+	if ( $is_performance_screen || $is_options_screen ) {
+		wp_register_performance_optimization_settings();
+	}
+}
+
+function wp_performance_filter_loading_optimization_attributes( $loading_attrs, $tag_name ) {
+	if ( wp_performance_optimization_enabled( 'lazy_loading' ) || ( 'img' !== $tag_name && 'iframe' !== $tag_name ) ) {
+		return $loading_attrs;
+	}
+
+	if ( isset( $loading_attrs['loading'] ) && 'lazy' === $loading_attrs['loading'] ) {
+		unset( $loading_attrs['loading'] );
+	}
+
+	return $loading_attrs;
+}
+
+function wp_performance_filter_image_editor_output_format( $output_format ) {
+	if ( ! wp_performance_optimization_enabled( 'image_optimization' ) ) {
+		return $output_format;
+	}
+
+	$modern_format = '';
+	if ( wp_image_editor_supports( array( 'mime_type' => 'image/avif' ) ) ) {
+		$modern_format = 'image/avif';
+	} elseif ( wp_image_editor_supports( array( 'mime_type' => 'image/webp' ) ) ) {
+		$modern_format = 'image/webp';
+	}
+
+	if ( ! $modern_format ) {
+		return $output_format;
+	}
+
+	foreach ( array( 'image/jpeg', 'image/png' ) as $mime_type ) {
+		$output_format[ $mime_type ] = $modern_format;
+	}
+
+	return $output_format;
+}
+
+function wp_get_performance_cache_dir( $type = '' ) {
+	$dir = WP_CONTENT_DIR . '/cache/performance';
+
+	if ( $type ) {
+		$dir .= '/' . sanitize_key( $type );
+	}
+
+	return $dir;
+}
+
+function wp_delete_performance_cache( $type = '' ) {
+	$dir = wp_get_performance_cache_dir( $type );
+
+	if ( ! is_dir( $dir ) ) {
+		return true;
+	}
+
+	$items = new RecursiveIteratorIterator(
+		new RecursiveDirectoryIterator( $dir, FilesystemIterator::SKIP_DOTS ),
+		RecursiveIteratorIterator::CHILD_FIRST
+	);
+
+	foreach ( $items as $item ) {
+		if ( $item->isDir() ) {
+			rmdir( $item->getPathname() );
+		} else {
+			unlink( $item->getPathname() );
+		}
+	}
+
+	return rmdir( $dir );
+}
+
+function wp_clean_performance_page_cache() {
+	wp_delete_performance_cache( 'page' );
+}
+
+function wp_can_optimize_performance_output() {
+	if (
+		(
+			! wp_performance_optimization_enabled( 'page_cache' ) &&
+			! wp_performance_optimization_enabled( 'minify_assets' ) &&
+			! wp_performance_optimization_enabled( 'critical_css' )
+		) ||
+		is_admin() ||
+		is_user_logged_in() ||
+		wp_doing_ajax() ||
+		wp_is_json_request() ||
+		is_feed() ||
+		is_robots() ||
+		is_trackback() ||
+		( defined( 'DONOTCACHEPAGE' ) && DONOTCACHEPAGE )
+	) {
+		return false;
+	}
+
+	if ( isset( $_SERVER['REQUEST_METHOD'] ) && 'GET' !== $_SERVER['REQUEST_METHOD'] ) {
+		return false;
+	}
+
+	$has_logged_in_cookie      = ! empty( $_COOKIE[ LOGGED_IN_COOKIE ] );
+	$has_comment_author_cookie = ! empty( $_COOKIE[ 'comment_author_' . COOKIEHASH ] );
+	$has_preview               = ! empty( $_GET['preview'] );
+
+	if ( $has_logged_in_cookie || $has_comment_author_cookie || $has_preview ) {
+		return false;
+	}
+
+	return (bool) apply_filters( 'wp_can_optimize_performance_output', true );
+}
+
+function wp_can_use_performance_page_cache() {
+	if ( ! wp_performance_optimization_enabled( 'page_cache' ) || ! wp_can_optimize_performance_output() ) {
+		return false;
+	}
+
+	return (bool) apply_filters( 'wp_can_use_performance_page_cache', true );
+}
+
+function wp_get_performance_page_cache_file() {
+	$url = home_url( add_query_arg( null, null ) );
+
+	return wp_get_performance_cache_dir( 'page' ) . '/' . md5( $url ) . '.html';
+}
+
+function wp_start_performance_page_cache() {
+	if ( ! wp_can_optimize_performance_output() ) {
+		return;
+	}
+
+	$cache_file = wp_get_performance_page_cache_file();
+	$ttl        = (int) apply_filters( 'wp_performance_page_cache_ttl', HOUR_IN_SECONDS );
+
+	if ( wp_can_use_performance_page_cache() && is_readable( $cache_file ) && filemtime( $cache_file ) > time() - $ttl ) {
+		header( 'X-WP-Performance-Cache: HIT' );
+		readfile( $cache_file );
+		exit;
+	}
+
+	if ( wp_performance_optimization_enabled( 'page_cache' ) ) {
+		header( 'X-WP-Performance-Cache: MISS' );
+	}
+
+	ob_start( 'wp_capture_performance_page_cache' );
+}
+
+function wp_capture_performance_page_cache( $output ) {
+	if ( ! wp_can_optimize_performance_output() || 200 !== http_response_code() || false === stripos( $output, '<html' ) ) {
+		return $output;
+	}
+
+	if ( wp_performance_optimization_enabled( 'critical_css' ) ) {
+		$output = wp_add_performance_critical_css( $output );
+	}
+
+	if ( wp_performance_optimization_enabled( 'minify_assets' ) ) {
+		$output = wp_minify_performance_output( $output );
+	}
+
+	if ( ! wp_can_use_performance_page_cache() ) {
+		return $output;
+	}
+
+	$cache_dir = wp_get_performance_cache_dir( 'page' );
+	if ( wp_mkdir_p( $cache_dir ) ) {
+		$cache_file = wp_get_performance_page_cache_file();
+		$tmp_file   = tempnam( $cache_dir, 'wp-performance-' );
+
+		if ( $tmp_file && file_put_contents( $tmp_file, $output, LOCK_EX ) ) {
+			rename( $tmp_file, $cache_file );
+		}
+	}
+
+	return $output;
+}
+
+function wp_add_performance_critical_css( $html ) {
+	if ( false !== strpos( $html, 'id="wp-critical-css"' ) ) {
+		return $html;
+	}
+
+	if ( ! preg_match_all( '#<style\b[^>]*>(.*?)</style>#is', $html, $matches ) ) {
+		return $html;
+	}
+
+	$critical_css = '';
+	foreach ( $matches[1] as $css ) {
+		$critical_css .= trim( wp_minify_performance_css( $css ) );
+		if ( strlen( $critical_css ) >= 12000 ) {
+			break;
+		}
+	}
+
+	if ( '' === trim( $critical_css ) ) {
+		return $html;
+	}
+
+	$style = '<style id="wp-critical-css">' . substr( $critical_css, 0, 12000 ) . '</style>';
+
+	return preg_replace( '#</head>#i', $style . '</head>', $html, 1 );
+}
+
+function wp_minify_performance_output( $html ) {
+	$html = preg_replace_callback(
+		'#<style\b([^>]*)>(.*?)</style>#is',
+		static function ( $matches ) {
+			return '<style' . $matches[1] . '>' . wp_minify_performance_css( $matches[2] ) . '</style>';
+		},
+		$html
+	);
+
+	$html = preg_replace_callback(
+		'#<script\b([^>]*)>(.*?)</script>#is',
+		static function ( $matches ) {
+			if ( preg_match( '/\btype=(["\'])(?!text\/javascript|application\/javascript|module)/i', $matches[1] ) ) {
+				return $matches[0];
+			}
+
+			return '<script' . $matches[1] . '>' . wp_minify_performance_js( $matches[2] ) . '</script>';
+		},
+		$html
+	);
+
+	$html = preg_replace( '/<!--(?!\[if|\s*wp:).*?-->/s', '', $html );
+	$html = preg_replace( '/>\s+</', '><', $html );
+
+	return trim( $html );
+}
+
+function wp_minify_performance_css( $css ) {
+	$css = preg_replace( '#/\*.*?\*/#s', '', $css );
+	$css = preg_replace( '/\s+/', ' ', $css );
+	$css = preg_replace( '/\s*([{}:;,>])\s*/', '$1', $css );
+
+	return trim( $css );
+}
+
+function wp_minify_performance_js( $js ) {
+	$js = preg_replace( '#/\*.*?\*/#s', '', $js );
+	$js = preg_replace( '/\s+/', ' ', $js );
+
+	return trim( $js );
+}
diff --git a/src/wp-settings.php b/src/wp-settings.php
index b2736bddadc3c..d8ab9c7d02421 100644
--- a/src/wp-settings.php
+++ b/src/wp-settings.php
@@ -261,6 +261,7 @@
 require ABSPATH . WPINC . '/class-wp-oembed.php';
 require ABSPATH . WPINC . '/class-wp-oembed-controller.php';
 require ABSPATH . WPINC . '/media.php';
+require ABSPATH . WPINC . '/performance.php';
 require ABSPATH . WPINC . '/http.php';
 require ABSPATH . WPINC . '/html-api/html5-named-character-references.php';
 require ABSPATH . WPINC . '/html-api/class-wp-html-attribute-token.php';
diff --git a/tests/phpunit/tests/ajax/wpAjaxWidgetsOrder.php b/tests/phpunit/tests/ajax/wpAjaxWidgetsOrder.php
new file mode 100644
index 0000000000000..15dd6534e4e9d
--- /dev/null
+++ b/tests/phpunit/tests/ajax/wpAjaxWidgetsOrder.php
@@ -0,0 +1,29 @@
+<?php
+/**
+ * Admin Ajax functions to be tested.
+ */
+require_once ABSPATH . 'wp-admin/includes/ajax-actions.php';
+
+/**
+ * Testing Ajax widgets order functionality.
+ *
+ * @group ajax
+ *
+ * @covers ::wp_ajax_widgets_order
+ */
+class Tests_Ajax_wpAjaxWidgetsOrder extends WP_Ajax_UnitTestCase {
+
+	/**
+	 * Tests that a missing sidebars payload returns -1 without notices.
+	 */
+	public function test_widgets_order_missing_sidebars() {
+		$this->_setRole( 'administrator' );
+
+		$_POST['savewidgets'] = wp_create_nonce( 'save-sidebar-widgets' );
+
+		$this->expectException( 'WPAjaxDieStopException' );
+		$this->expectExceptionMessage( '-1' );
+		$this->_handleAjax( 'widgets-order' );
+	}
+}
+
diff --git a/tests/phpunit/tests/performance.php b/tests/phpunit/tests/performance.php
new file mode 100644
index 0000000000000..0ff1fae10f850
--- /dev/null
+++ b/tests/phpunit/tests/performance.php
@@ -0,0 +1,98 @@
+<?php
+/**
+ * Tests for native performance optimization functions.
+ *
+ * @package WordPress
+ * @subpackage UnitTests
+ */
+
+/**
+ * Tests performance optimization helpers.
+ *
+ * @group performance
+ */
+class Tests_Performance extends WP_UnitTestCase {
+	/**
+	 * Cleans up settings after each test.
+	 */
+	public function tear_down() {
+		delete_option( 'performance_optimization' );
+		parent::tear_down();
+	}
+
+	/**
+	 * Tests sanitizing performance settings.
+	 */
+	public function test_wp_sanitize_performance_optimization_settings_preserves_defaults() {
+		$settings = wp_sanitize_performance_optimization_settings(
+			array(
+				'page_cache'    => '1',
+				'lazy_loading'  => '0',
+				'unknown_value' => true,
+			)
+		);
+
+		$this->assertTrue( $settings['page_cache'] );
+		$this->assertFalse( $settings['lazy_loading'] );
+		$this->assertFalse( $settings['minify_assets'] );
+		$this->assertArrayNotHasKey( 'unknown_value', $settings );
+	}
+
+	/**
+	 * Tests disabling lazy loading removes only lazy loading attributes.
+	 */
+	public function test_wp_performance_filter_loading_optimization_attributes_removes_lazy_loading_when_disabled() {
+		update_option(
+			'performance_optimization',
+			array(
+				'lazy_loading' => false,
+			)
+		);
+
+		$attributes = wp_performance_filter_loading_optimization_attributes(
+			array(
+				'loading'       => 'lazy',
+				'fetchpriority' => 'high',
+			),
+			'img'
+		);
+
+		$this->assertArrayNotHasKey( 'loading', $attributes );
+		$this->assertSame( 'high', $attributes['fetchpriority'] );
+	}
+
+	/**
+	 * Tests image output mappings are unchanged while image optimization is disabled.
+	 */
+	public function test_wp_performance_filter_image_editor_output_format_returns_existing_map_when_disabled() {
+		$output_format = array(
+			'image/heic' => 'image/jpeg',
+		);
+
+		$this->assertSame( $output_format, wp_performance_filter_image_editor_output_format( $output_format ) );
+	}
+
+	/**
+	 * Tests critical CSS generation from inline styles.
+	 */
+	public function test_wp_add_performance_critical_css_adds_inline_block() {
+		$html = '<html><head><style>.site { color: red; }</style></head><body></body></html>';
+
+		$this->assertStringContainsString(
+			'<style id="wp-critical-css">.site{color:red;}</style>',
+			wp_add_performance_critical_css( $html )
+		);
+	}
+
+	/**
+	 * Tests output minification.
+	 */
+	public function test_wp_minify_performance_output_minifies_inline_css_and_html() {
+		$html = "<html>\n<head><style>.site { color: red; }</style></head>\n<body><!-- remove me --><p>Test</p></body>\n</html>";
+
+		$this->assertSame(
+			'<html><head><style>.site{color:red;}</style></head><body><p>Test</p></body></html>',
+			wp_minify_performance_output( $html )
+		);
+	}
+}