Rocket loader

Rocket Loader prioritizes your website's content (text, images, fonts etc) by deferring the loading of all of your JavaScript until after rendering.

This type of loading (known as asynchronous loading) leads to the earlier rendering of your page content. Rocket Loader handles both inline and external scripts while maintaining the order of execution.

If you've elements in the above fold that require JavaScript to render, it's better to disable Rocket Loader.