I know back at the beginning of the year it was caused by wiki pages not having a timeout, so it shouldn't be that, and I know that it shouldn't be saved searches causing the issue because of topic #25290.
As with last time, it tends to affect searches of older images more often, and usually only when you get a few hundred pages in. I'm also occasionally getting "ActiveRecord::QueryCanceled" error messages in some of my API-driven content scans, which makes me think there's something going on with the database itself.