Async JavaScript vs Speed Up – Optimize CSS Delivery

Eliminate Render-blocking Javascript in above-the-fold content with Async Javascript. Render-blocking Javascript prevents above-the-fold content on your page from being rendered until the javascript has finished loading. This can impact on your page speed and ultimately your ranking within search engines. It can also impact your user’s experience. Async JavaScript gives you full control of which scripts to add an ‘async’ or ‘defer’ attribute to or to exclude to help increase the performance of your WordPress website.

This plugin is no longer recommended. Deferring every stylesheet makes the page paint before its own layout arrives. That flash of unstyled content is measured as Cumulative Layout Shift — one of the Core Web Vitals this plugin is meant to improve. The current advice is to inline the CSS needed for the top of the page and defer only the rest, which is more than rewriting a link tag can do. If you have not configured the exclusion filter for your main stylesheets, deactivating this plugin will most likely improve your scores rather than worsen them. This small plugin (5 Kb) loads the stylesheets asynchronously and improve page load times. The recommended use of this plugin is to load your vital stylesheets synchronously and non-vital CSS files asynchronously. Non-vital CSS-files can be for example: fonts, icons, before the fold template-specific CSS, etc. You can choose which files to load synchronously with a filter in your function.php, eg.: // exclude main and child stylesheets from delivery optimization function exclude_from_delivery_optimization($handle){ return in_array($handle, array('main-stylesheet', 'child-stylesheet')); } add_filter('speed-up-optimize-css-delivery', 'exclude_from_delivery_optimization'); Note: this only works if your other plugins and theme add the CSS correctly.