what is the best forum software

[Matt]

I've always worried about phpBB having security issues and being plagued with spam. Besides, I'm not too much a fan of the design of the thing.

MyBB is very customizable (provided you have a bit of CSS knowledge) to fit your needs, and is really quick. The server, as Vanilla said, is what basically can make or break your speed. NiftyHost's things are pretty fast, I use a different host though. If you have the money to spend, I would recommend hosting some static content from your forum on a CDN (content delivery network) such as Rackspace (http://rackspace.com)'s Cloud or Media Temple (http://mediatemple.net)'s ProCDN solution. It'll seriously improve site speed. Other small factors like DNS settings influence this, too- and IntoDNS.com can help with diagnosing these issues. Google Page Speed can also give helpful suggestions on how to improve your site's speed.

Hope it helps!