I have made a temporary hack (NOT to be CVS committed) to the forums threads summary page to speed the non-cached query by a whole bunch (a couple of seconds to load now rather than 18).
I've had to disable caching of rows for that page temporarily but I think the faster average time and losing the horribly slow worst case after each post is worth it.
This is not related to fixing list builder's pagination ... strictly a short-term hack.