Chances are you are using at least one javascript library or css framework that is on other websites as well. If you host these files yourself then the browser assumes that your version of jQuery, for example, is different than the version on some other site and downloads it again. If you use a Public CDN not only do you get the usual benefits of of a CDN but the browser will realize your library is the same as the one it already downloaded on another site that uses that CDN. This means it doesn't have to request the file again and fewer request means a faster site.