At work I'm doing some researching and setting up of things to help improve the front-end performance of our stuff. Here are two quick things that keep bubbling up.
Prefer fewer, larger, files
Avoid many, smaller, files.
This is because, broadly speaking, latency trumps bandwidth when it comes to performance concerns.
Prefer simple CSS selectors
Rather have an extra class in your HTML.
This is because browsers can figure out simpler selectors faster.