Site slowness.

The site is frequently running ridiculously slowly. At present I don't really know why. Sometimes it's obvious (load average 30, Apache/PHP compiling lots of slow queries running), sometimes it isn't (when it was slow this afternoon). The system still has plenty of memory free. The squids don't show an unusual rate of hits on the site. Further updates at RationalWiki:Technical support. At least we can all be puzzled in a fully informed manner.