Just to recommend Jerome’s Query Diagnostics plugin for Wordpress, which once enabled spits out a list of all the database queries used by a page, along with the time they took to complete. This afternoon I found that one plugin was doubling the processing time, and removing that has made the whole site snappier.  I was trying to do this by looking at mysql logs, but it seems such things don’t really exist on a per-user basis, so anybody sharing a server with x number of others is going to struggle to diagnose connection problems. People recommend other plugins, but JQD was the only one that worked for me.