Prefer Asynchronous Resources

As you are aware, as web pages load, the traditional route is to load resources one by one, or synchronously. As the browser parses HTML, everything from CSS to various scripts are rendered and loaded. The problem with this method is that if the browser reaches a script that is not needed during the initial page load, then everything stops until that script is downloaded, which causing page load delays. On the other hand, with asynchronous loading, if the browser encounters any JavaScript, it will not delay the rendering of other web page elements. This gives the user the impression that everything is loading quickly instead of a blank page. Another reason for considering asynchronous loading is that it loads the page faster, which is good from an SEO perspective. Faster page speed can lead to better rankings in the SERPs.

For all the benefits that asynchronous loading provides, there are recommendations to not implement it in certain circumstances, like if the scripts need to be loaded in a specific order, then do not utilize asynchronous loading. An important note is that asynchronous loading does mean that content is loaded first and then the scripts at the end. That is a different optimization technique we discussed earlier in this article called deferring the parsing of JavaScript, which does exactly that, however, you also want to be careful how you defer the parsing of JavaScript. You do not want to move all your scripts to the end, as from the user perspective, it will look like the page is still loading and they may not interact with page until it completely loads. Deferring the loading of non-essential JavaScript until the end creates better page load performance, however, it is a careful balancing act and must be done properly to ensure an efficient experience.