Well, I dont see why would AT not have mod_gzip installed(according to
http://leknor.com/code/gziped.php). It would reduce the text download time drastically without increasing the cpu loads by much.
I am sure that the sloweness of the forums is not only due to the fact that AT is so popular, but also that they are trying to sell the subscribers forums. So its not neccessarily due to technical issues that free version of AT is slow, but hey, you get what you paid for