Here's the deal:
As far as decent forum software goes, there's not a lot that's actively developed anymore. SMF kinda sucks, but it kinda sucks the least.
One of the problems it has, is scaling issues - meaning the more people using it, the slower it gets. Until they update it to be usable with the newest version of PHP (the programming language it's made in) it's pretty taxing on a server.
All I can do to mitigate the speed is throw more money at it and give us more RAM/CPU speed.