Sometimes I wonder what was wrong with a mostly static pages in the first place. Even http://mail.google.com/mail/h/ works fine, even in a slower browser - and I would say 80% of the sites today are less complex than Gmail.
I forgot how great Gmail was before they started doing major redesigns to it, although I do hate to say it... but it could benefit from a little JS to dynamically update with new e-mails...