Probably a bit difficult to do that, when usually resource management only limits memory and/or processor usage. The thing that would be slowing down the forums that much would be disk access. We post a lot
Maybe the backups can be changed, so that, say, once a month, the entire forums are dumped to backup. Then, to keep up to date, the nightly backups only backup posts between specific times. So, you would have the total backup for January, then on February 1, all the posts made from midnight to midnight EST would be backed up, repeat for each day of the month. Then, in the event that the backups are needed, just use the lastest monthly backup, then apply each daily backup to restore the forums fully. It increases by a bit the time needed to restore the forums, but decreases the time needed to backup dramatically.