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.
Basically, if someone never reads a forum, every time they visit it has to figure out how many unread posts are in each forum. As the number of topics and posts grow, the number of rows that are scanned grows.
If all of your users regularly visit each forum, the queries are pretty quick. But if they only visit a few of the forums, the number of unread posts in the other forums grow, leading to larger query times.
One of the main reasons you don't see this feature on many other forums, is there is no non-resource intensive way to do it. We kept it in as an option because some people love it, but it's a resource hog, plain and simple
Forum Display Options has another one of these features with the same warning.
