For a large forum there's not much you can do, besides pruning (which I hate to do, I still have stuff from 1999
). It sounds like it's time to upgrade hosting, and/or move to threads 7 - the caching is bound to make it easier on the server. Keep in mind the mysql(4+)/php(4.3+) requirements for threads 7 if you upgrade, your older server probably doesn't run current software.