Let me guess. You have the "Show total # of unread posts per forum?" checked under Forum Summary Options. Where right under it, it says:
Warning: This can cause excess load and contribute to slow speed.
Well, I disabled both about 26 hours ago and {knock on wood} haven't had a single lag issue since, so thanks!
...but some of my members are bitching about it being gone.
I may try one or the other and see how that works out as a compromise. But in any event - that seems to have made a big difference, so thanks!
