When they upgraded to the database driven board instead of the old PERL script version, it greatly improved the board speed. I don't think that the word association thread is killing too much, because it only queries the database for it when it is called. Even then, it only calls the page you're looking at.
Small stopgap solutions could be lowering the amount of topics displayed on the front page and to a lesser degree the number of posts per page when you click on a thread.
GIK, do you have any way of finding out how much bandwidth you are actually using on an average month?