According to _ck_ bbPress is faster. According to Sam phpBB is “probably” faster in some situations, in others bbPress is faster. In most situations I’d assume bbPress would load paages faster simply because it generally has a lot less to do, both content wise (less features = less HTML/CSS) and processing wise. The only way to tell would be to bench test two identical setups, but I haven’t heard of anyone doing that before.
In most practical situations I doubt the performance issues are a real issue though. The way you code your theme will probably have more effect on page load times than anything in the backend.