js-async-loader.php

//doplní všem skriptům, na kterých nejsou jiné závislé atributy async a defer

if (!is_admin()) {
  add_filter('script_loader_tag', 'lynt_async_attr', 10, 3);
}

function lynt_async_attr($tag, $handle, $src)
		{
			global $wp_scripts;

			$dependency = false;
                        $blacklist = ['smush-lazy-load'];

			foreach ($wp_scripts->registered as $script) {
				if (in_array($handle, $script->deps)) {
					$dependency = true;
					break;
				}
			}


			if ( !$dependency && !in_array($handle, $blacklist) ) {
				return str_replace('src=', 'async defer src=', $tag);
			}
			return $tag;

		}