Typically, JavaScript files loaded with the tag, block the download of resources as well as the rendering of elements within the web page. So, even though most modern browsers tend to support the parallel download of JavaScript files, image downloads and page rendering still have to wait for the scripts to finish loading. In turn, the amount of time a user has to wait for the page to display increases.

This is where asynchronous loaders come in to play. Using one of several different load techniques, they remove the inherient blocking nature of a script, which allows for parallel downloading of both the JavaScripts and resources while not interfering with page rendering. In many cases, this can reduce – sometimes drastically – page load times.

Asynchronous loading of scripts on a webpage.