Ok, final verdict: CS3=no slowdown. yay!
And it does what I need it to do, so I'll be sticking with this, probably for the next 5 years or so (that's about how long it's been since I first got 7, got my moneys worth out of that copy, let me tell ya) (in fact, i probably would still be using 7 with no complaints if not for the pathing for the text, and that 90% of the brushes I got from the one CD wouldn't work with 7)
So, saga complete. Thanks for the suggestions to all who suggested, it really did help narrow down the issue (and rule out other possibilities I hadn't considered)