If you haven’t tried the new Gmail with a recent nightly WebKit (you need build 523 or later due to one blocking bug in earlier builds) I dare you to try it. With nightly WebKits it takes about 200ms to go from the Inbox to a thread. Anything that takes less than 300ms is perceived as instant and it should be the goal of all user initiated actions in an application.
Once you feel the speed of Gmail + WebKit it is hard to beleive that anyone would want to use anything as slow as the alternatives.