Today there are still limits in downloading files from the web server to the local browser.
a- as the browser can only make a limited number of requests at one time you have to limit them.
put all JS and CSS together in less files possible.
b- compress the code. this is a slight saving, but every kb helps especially if you consider the cost of bandwidth on a multi-user app
c- use sprites combining multiple images into a single image.
I still have to experiment with this option
e- download the yslow plugin for firefox to test and check some other features.