I occasionally have to refresh a page cause gmail gets stuck on a loading screen but apart from that it's pretty quick, my inbox isn't that large though. Looking forward to POP access!
I've found that if you reply but remove the previous messages from the reply it speeds things up quite nicely and they remain threded... I had one going 40 deep and realized I was actually sending our whole conversation back to the recipient (who was also using gmail and sending me the whole conversation). That helped a ton with speed.
I have the same problem as a poster above where it gets stuck on a loading screen. Otherwise I think the speed is fine. POP access would be nice - cuz I can then use Mail. And I'm looking forwards to the new Mail in Tiger!