On a page with independent photoshop benchmarks ( http://www.driverheaven.net/photoshop/ ) , they found out that many filters are faster in each new version of Photoshop: optimizations are being added for new instructions in newer processors.
The only cases where a filter/operation slows down is when the engineers remarked that the transformation was not "clean", and they decide to make some code that might be more accurate, but slightly slower, but such a case is very rare.
Of course, if you stay with the same processor for many versions, you won't profit of the new optimizations that might be limited to SSE2 or 3, multi-threading, and in the future, 64bits coding.
In one of the NAPP vidcasts, one saw for a few seconds an engineer that works specifically on startup time. It is not because there are a lot of new functions that the whole app will take longer to load on memory. In several cases, it is the number of fonts, presets, textures, plug-ins that can slow down startup time.
I know that I may sound like an evangelist, but a lot of visits to the official Adobe Forums, and many other resources taught me those tidbits.
