Well, so much for that approach. It was pegged at 100% CPU for a coupla hours tonight before I restarted mysqld. (Only this time ubbt_POSTS was marked as needing repairs).

Now to go look up how to turn on logging of slow queries.

I don't mean to interrupt, but we've had similar symptoms on my board for the past few months. Everything had been fine, but now the system hangs every few days (lots of Apache threads waiting). Eventually, it just fixes itself. I don't have the SQL details as bakerzdosen does. I have checked the settings described above and they're set correctly. We have a dedicated server. I don't know SQL commands well enough to try to diagnose when a problem is occurring, though.

I have a large number of posts (over 800,000). Is there some reorg command I could issue?

My users have had similar complaints in the last few months. I checked and it seems to be mysqld which is choking. We have a relatively small forum too.

I'm not sure if this helps or not, but it seems to be worse on the main forum page. by clicking on "Forum List", here is an example of the slowness in debug info:

The front page will normally generate about 9-10 queries if all posts are read. So when you see things like 23 queries total, it means that you probably have full unread post tracking turned on, and the user in question has about 13-14 forums with unread posts. Each one of those queries is pretty expensive so it can add up and slow things down pretty quickly.

I've not looked into it but on another site with different software they were actually having a recursive lookup issue that really slammed the board the more users they had.

Hopefully UBB doesn't have that issue in any of it's code but also make sure any home or custom pages you've created don't have that issue.