What are the real-world performance gains of having a photoshop scratch disk?

"I remember when it used to be very useful, typically when the files people were editing were the size of half their memory (or more) but now the memory has become more and more and the photos haven't become that much bigger, I see it as less and less important.

Not saying you won't see any speed improvements, but if you are just a casual user with a machine that has around 4gb of memory, and your output files are under 100Megs, I would say that you may not see any speed imporvements."

Check out this presentation written by one of the Adobe co-architects. Talks about many issues related to performance tuning, but as far as file IO goes, having a dedicated disk prevents other resources from competing with Photoshop.