On an account with a reasonable number of posts, the spam cleaner took about an hour and ten minutes to run. This is much longer than the PHP timeout, so it looked like it had just died, but actually it was still running, and maintaining a lock on the forum database, for that time. Then it completed.
For science reasons, I went to undo one of the old spam-cleaner runs. It's been going for something like an hour so far. Process is still running, mysql is still showing load. (This time I had the foresight to take the forum down first.)
So, I'm wondering if there's any more efficient way to hide a user. Goal would be, all of their posts disappeared and no longer visible, but restorable later.
(If you're wondering why: Because sometimes the abuser shows up, and we don't want them getting to look through all the stuff said about them.)