Riklaunim was nice enough to post some Benchmarks for different forum, blog, and CMS systems.

Surprise surprise!
punBB is the fastest/stable forum system, followed by phpBB 2.x (Yay!), then SMF and IPB 2. (Actually, punBB was the fastest system tested period!)

Also, tested are Drupal, Wordpress, Joomla, phpNuke, and others. So go check it out!

Yeah! I use punBB for my forum - and yep! I like it.

Of course, I chose to use punBB because it was the easiest one for me to set up.