wouldn't everything run better if ran "under optimum server configs"
it really depends on the amount of hacks and hacking you do to your forum.. vBulletin is a paid script, it will obviously run better then a free one. There are plenty of big boards running vbulletin, as well as many running phpbb.. It's really on personal prefrence, you can run a successful site with phpbnb, but I would sure rather update a vBulletin forum, vs a phpbb one, that's for sure