An optimized admin-ajax.php that executes visitor-only AJAX requests in WordPress

admin-ajax.php

<?php
// Note: This file exists as a near-copy of /wp-admin/admin-ajax.php so that caching on this URL can occur.

/**
 * WordPress AJAX Process Execution.
 *
 * @package WordPress
 * @subpackage Administration
 *
 * @link https://codex.wordpress.org/AJAX_in_Plugins
 */

/**
 * Executing AJAX process.
 *
 * @since 2.1.0
 */
define( 'DOING_AJAX', true );
if ( ! defined( 'WP_ADMIN' ) ) {
	define( 'WP_ADMIN', false );
}

// Fake being the real admin-ajax.php so some WordPress functions run correctly.
$_SERVER['PHP_SELF'] = '/wp-admin/admin-ajax.php';

/** Load WordPress Bootstrap */
$str_location_wp_load = dirname( dirname( dirname( dirname( __FILE__ ) ) ) ) . '/wp-load.php';
if ( !file_exists($str_location_wp_load) ) { // Handle local development environments that are using symlinks.
	$str_location_wp_load = $_SERVER['DOCUMENT_ROOT'] . '/wp-load.php';
}
require_once( $str_location_wp_load );

/** Allow for cross-domain requests (from the frontend). */
send_origin_headers();

// Require an action parameter
if ( empty( $_REQUEST['action'] ) )
	die( '0' );

/** Load WordPress Administration APIs */
require_once( ABSPATH . 'wp-admin/includes/admin.php' );

/** Load Ajax Handlers for WordPress Core */
require_once( ABSPATH . 'wp-admin/includes/ajax-actions.php' );

@header( 'Content-Type: text/html; charset=' . get_option( 'blog_charset' ) );
@header( 'X-Robots-Tag: noindex' );

send_nosniff_header();
nocache_headers();

/** This action is documented in wp-admin/admin.php */
// do_action( 'admin_init' ); // Disable as we'll never be using this file to do things only for logged-in users.

$core_actions_get = array(
	'fetch-list', 'ajax-tag-search', 'wp-compression-test', 'imgedit-preview', 'oembed-cache',
	'autocomplete-user', 'dashboard-widgets', 'logged-in',
);

$core_actions_post = array(
	'oembed-cache', 'image-editor', 'delete-comment', 'delete-tag', 'delete-link',
	'delete-meta', 'delete-post', 'trash-post', 'untrash-post', 'delete-page', 'dim-comment',
	'add-link-category', 'add-tag', 'get-tagcloud', 'get-comments', 'replyto-comment',
	'edit-comment', 'add-menu-item', 'add-meta', 'add-user', 'closed-postboxes',
	'hidden-columns', 'update-welcome-panel', 'menu-get-metabox', 'wp-link-ajax',
	'menu-locations-save', 'menu-quick-search', 'meta-box-order', 'get-permalink',
	'sample-permalink', 'inline-save', 'inline-save-tax', 'find_posts', 'widgets-order',
	'save-widget', 'delete-inactive-widgets', 'set-post-thumbnail', 'date_format', 'time_format',
	'wp-remove-post-lock', 'dismiss-wp-pointer', 'upload-attachment', 'get-attachment',
	'query-attachments', 'save-attachment', 'save-attachment-compat', 'send-link-to-editor',
	'send-attachment-to-editor', 'save-attachment-order', 'heartbeat', 'get-revision-diffs',
	'save-user-color-scheme', 'update-widget', 'query-themes', 'parse-embed', 'set-attachment-thumbnail',
	'parse-media-shortcode', 'destroy-sessions', 'install-plugin', 'update-plugin', 'press-this-save-post',
	'press-this-add-category', 'crop-image', 'generate-password', 'save-wporg-username',
);

// Deprecated
$core_actions_post[] = 'wp-fullscreen-save-post';

// Register core Ajax calls.
if ( ! empty( $_GET['action'] ) && in_array( $_GET['action'], $core_actions_get ) )
	add_action( 'wp_ajax_' . $_GET['action'], 'wp_ajax_' . str_replace( '-', '_', $_GET['action'] ), 1 );

if ( ! empty( $_POST['action'] ) && in_array( $_POST['action'], $core_actions_post ) )
	add_action( 'wp_ajax_' . $_POST['action'], 'wp_ajax_' . str_replace( '-', '_', $_POST['action'] ), 1 );

add_action( 'wp_ajax_nopriv_heartbeat', 'wp_ajax_nopriv_heartbeat', 1 );

if ( is_user_logged_in() ) {
	/**
	 * Fires authenticated AJAX actions for logged-in users.
	 *
	 * The dynamic portion of the hook name, `$_REQUEST['action']`,
	 * refers to the name of the AJAX action callback being fired.
	 *
	 * @since 2.1.0
	 */
	do_action( 'wp_ajax_' . $_REQUEST['action'] );
} else {
	/**
	 * Fires non-authenticated AJAX actions for logged-out users.
	 *
	 * The dynamic portion of the hook name, `$_REQUEST['action']`,
	 * refers to the name of the AJAX action callback being fired.
	 *
	 * @since 2.8.0
	 */
	do_action( 'wp_ajax_nopriv_' . $_REQUEST['action'] );
}
// Default status
die( '0' );

Dare to compare against the official file at https://github.com/WordPress/WordPress/blob/master/wp-admin/admin-ajax.php . Differences include:

• WP_ADMIN is set to false instead of true
• admin_init actions are not executed (the do_action() call is commented out)

This file needs to be carefully examined each time you upgrade WordPress, but often speeds up AJAX requests by 50% or more by skipping the execution of admin-side WordPress actions and libraries.