One of the stated reasons Adobe did the beta release of Photoshop CS3 was to provide native support for the newer Intel processor Macs. So it is logical and was my experience that Photoshop CS 3 beta ran much faster on an Intel Mac, and not nearly as fast on a Dual Power PC G5. So the conclusion that is logical is that the optimization to run Intel Macs native has the opposite affect on older Power PC machines, at least my experience with the beta indicated as much.

So, for the functions that mentioned and you find run slow, run them on CS2 as no advantage is lost by not using CS3, and only use CS3 for those new features that are an advantage and not available with CS2, at least until you can upgrade your hardware to an Intel based Mac.