I'd happily take the current version, though 10 times faster.
I've tried the speed on several computers, home (8Mb) and work (OC-3) on several different OSs and computers. Access to the inbox appears to take ~20-30 secs, irrelevant of how much mail the inbox holds. Subsequent actions (view email etc.) appear to take approx. 10+ seconds. Friends with Zen also report the same issue.
I know the interface is a little lacking at the moment - but hey, it *is* useable. (Though I only have a couple of hundred emails at any one time).
Personally, I'd go for speed over UI any day. Is it just a matter of chucking faster/more servers at the system, or is the code that runs it a bit slow (presumably running security checks or suchlike)?
Best,
Jez