if( defined('DOING_AJAX') && DOING_AJAX && strcasecmp($_SERVER['REQUEST_METHOD'], "POST") == 0 && !empty($_POST['action']) && $_POST['action'] == "{your_action}") {
    $ajax_cache_path = WP_CONTENT_DIR . "/custom-cache/{$_SERVER['HTTP_HOST']}/cache_ajax/";
    $cache_key = ajax_cache_key(); //Generates a unique filename with sha1 based on information from the POST request (GET information is always the same in this case)

    if( file_exists($ajax_cache_path . $cache_key) ) {
        header('content-type: application/json; charset=UTF-8');
        header('AJAX-Cache: HIT');
        die( file_get_contents($ajax_cache_path . $cache_key));
    }

    ob_start( 'ajax_cache_end' );
}