when the css and js is minimalized, people should see a noticeable difference. That won't happen until 1.0 gold.
As mentioned, mybb uses TOO MUCH table code. Granted some should probably be used, but not that much.
And mybb is FUGLY.
xF has the best SEO of any forum software(by far, in my opinion) and that says alot. MyBB may be slightly faster, but xenForo is offering 3 times as much in my opinion. It's well worth the trade off...