I like Yahoo! Mail much better than Gmail in every, to me, important way. But in one area Gmail is better: Speed. Scrolling through mail, open new mail for editing, things like that, is much faster in Gmail. I'm not talking subtle differences here, but a very different feel. It's the same in all the browsers I have tried (FF, Opera and IE). Still, with this performance difference Yahoo! Mail is still the one for me, but I would like to know what the reason for this performance difference could be. Is it how the UI interacts with the server back-end? Is it the server back-end itself being slow, or is it the UI implementation which is simply too complex for today's web browsers?
And please do not try to tell me that there's no difference in performance between the two. There is! :)