that's not slowdown....
gt2 runs at 30fps, since the psx doesn't have the power to run it at 60fps.
I'd swith the pc fps calculation on if I were you, because otherwise you'll experience REAL slowdown, even though it reads 60fps.
the pc fps calculation differs in that way that it's some sort of speed hack to run certain games better. (pete could probably explain better than me)
fact is, the normal (psx calculation) will always display 60fps (if running full speed)...however, that's not completely the truth. GT2 is in fact running at half/rate, 30fps. In fact, probably most racing games you've played don't run faster than that. Only since DC and PS2 many racing games run REALLY at 60fps.