I have 7.1GB, it's still beyond fast. Size isn't a sufficient condition for whatever problem it is you are seeing. The only time I had a problem was before I abandoned greasemonkey.
Everything is slow. Sending mail, switching labels, searching, even clicking settings or the home icon. I've tried across lots of browsers and computers. It's related to the account. Other, much smaller and newer accounts are fast.
Which operations are slow for you?