I'm not sure if it's the Hud or the autoimporting, but when I have 24+ tables up on Pokerstars it lags very heavily. It probably starts having a noticeable effect around 12 tables. I have a very fast computer and PT2 + PAH has always worked fine. I'm starting to think it's the importing that's slowing everything down though because it reads in hands very slowly when autoimporting although manual imports are very fast. Perhaps there should be a feature that allows you to run the Hud without importing.
As a side note I do have about 12 stats per player on the Hud. I don't know how much that would affect it.