Well, I'm used to deal with very large vBulletin 3.8.x forums (50+ million posts or 30k+ online users) and performance was never an issue with the right optimizations and server hardware. So with XenForo should be even less demanding. Of course, you cannot run your forums on an Atom processor (figure of speech) and think HipHop will help. Obviously, for a site like Facebook, running HipHop results on substantial hardware savings. For a typical forum, is a waste of time and other technical resources cost to constantly maintain the software.