I have a good sized vBulletin board (1.4 million posts and 1.4 gigs database). There is not a terribly heavy amount of activity. Most of the posts are either old or come from a small but active group of members who post a lot. Server load is quite reasonable, usually close to 0 (currently 0.10). The problem is it's RAM hungry, easily chewing through the 32GB I had on my last server. This could be in part because of my MySQL settings, which more or less are set as recommended by vBulletin support, and/or because of some of the hacks I have installed. Anyway, I'm curious as to how Xenforo may compare.