[bitfolk] Checking VPS performance

Hello, Recently my VPS has begun performing certain IO intensive operations notable slower, in some cases maildirs that can take less than a second to open under ideal circumstances have been taking up to a minute to load. I am told by Andy that iowait on the host server (barbar) is not high at the moment, so it is likely to be something to do with the VPS itself. Can anyone suggest any diagnostic measures I can take to identify where the performance issue is? I have never had to deal with such a situation before, and Google is not offering a significant amount of guidance. Thanks, Robert