As web applications evolve to provide the rich user experience that was once the sole domain of desktop applications, their CSS and JavaScript files have grown in size and number. Since a web page won't even render under its CSS and JavaScript files have been loaded, each one of these files slows down the application a little.
The solution put forth by Wormly's site is simple: consolidate and cache. In other words, gather all your CSS into a single .css file and gather all your JavaScript into a single .js file. That way, only two resources are required before page rendering starts, and two simultaneous downloads is well within the limits of most browsers. Serving the necessary HTTP headers alongside these files encourages the browser and the proxy server to cache them aggressively, meaning more perfomance gains.