Sylpheed? Or Sylpheed-claws if you require more functionality, but sylpheed's IMAP handling is far superior, as I recall. I haven't noticed speed problems, but one annoying thing is that it seems to run network and gui code in the same thread, so the interface can stall on network problems (it does timeout, but it can be a few seconds). And it's below opera, X, firefox, and gaim in memory usage on my box right now.

I tried mutt for awhile, but I got rather annoyed by it being (adamantly) just an MUA and not an MTA. pine is awesome, too, but eh.

-Deason