One of the ways to speed up a website is by offloading static content to other server. Static content includes images, and JS and CSS files.
Although there are other script-hosting alternatives, such as Google Hosted Libraries, and Microsoft Ajax CDN, they only host most popular libraries. On the other hand, cdnjs has the most extensive, if not all, libraries.
If you're using CMS, such as WordPress or Joomla, you can use cdnjs plugin. For other platform, you can manually offload JS and CSS by linking to the appropriate files in cdnjs with the
link tags, respectively.