I'm a new xenforo admin with a new site - and I'm really impressed with it's ability to handle big surges in traffic and not decapitate a mediocre server.
I started a wordpress blog last year. I got bored with the blog format (I guess I'm old school!) and did what many consider to be heretical: I dumped wordpress for xenforo. My xenforo is setup as a hybrid blog/forum site. I and a few friends are the contributors. We are the only ones that can start new threads. (Another heretical forum practice!)
Other than google punishing me for changing my links and more, my traffic is quickly building again with periodic big traffic spikes which last a day or two. (These stats are "ho-hum" for some of you big-board admins - this discussion is how a minimally configured server can operate flawlessy with XenForo.)
So to get to brass tacks: The server is dedicated. I have several other sites running on it. It only has 2 Gigs of memory and a single 2.1'ish Ghz chip. The last traffic spike saw up to 15 requests per second, up to 468 live active people online (google analytics) with a sustained 200+ live and active people over a 16 hour period. Loads always stayed under 3! I was monitoring the load when we were running 400+ live sessions and the server was cruising along at a 2.42 load. At ~250 live sessions we were at a sub 1.00 load. I'm impressed!
If I was using my "other software", I would have been re-booting all the time.