Nettiquette is built into web browsers. When you go to download a page, its contents will load in at a max of 2 files at a time (by default). So if there are 12 CSS and JS files, you’ll only get 2 at a time until you load them all.

The good news is you can trick browsers into doing more at a time.

Enter subdomains.

Offload your CSS to css.yoursite.com, and your JavaScript files to js.yoursite.com. Now your website will load 6 files at a time (2 CSS files, 2 JavaScript files, 2 HTML/Image files from your main site).

It doesn’t matter if each subdomain is pointing to the same website. Web browsers go by the site URL and nothing more.

HTTP/2 is supposed to eliminate this problem, of course, but in the meantime doing this helps when you can’t crunch down your files any more.