It's not necessarily that they need to read every post. It kind of compounds. It looks like you have maybe 200 forums or so, give or take 10 or 20
When you have full post tracking turned on (the little red numbers next to the topics/posts) it works like this. If the last post time is greater than the user's last visit to that particular forum, it has to do a query to see how many topics/posts have been made since that last visit.
So, if you have 2 forums with unread posts, that's only a couple extra queries so that's not too bad. But say you have 150 forums with unread posts. That's 150 extra queries to load the front page. Unfortunately those aren't simple queries either and they can't be done in one big query. Each have to be done individually.
Honestly, with the number of forums you have I'd really consider turning off full post tracking.
