This looks like you have the option to show total # of unread topics/posts. You can find this under Display Options. Both of these can cause a pretty high load on the SQL server especially if you have quite a few users that have a lot of unread posts.
It has to go through each forum, checking their last visit, and tallying the number of posts that have been posted since then. What happens is some users just visit a couple forums, while never visiting some of the other ones, so the number that needs to be counted continues to grow.
You can encourage your users to frequently "Mark all as read" to help, but you may just need to turn the feature off.
