Phpbb works faster than yabb in my opinion, I do use gzip compression and have never had a complaint about a slow forum you can check it outhttp://www.trudyscandles.com/groupladder/forum I also think the mod developers are very high quality, I like to keep improving it...lol, the support forum is tops, nearly live help and many many questions answered.
I did however just install a Yabb forum athttp://www.trudyscandles.com/forum however it's not setup yet, I had it going with the latest yabb version and the same day they announced bugs "after the official release!" I went to the next earlier version 1.4.1 I think, and will just add the patches.
I do like the Phpbb though :)

Jack