Weird, my 3GS seems no different on iOS 5 vs iOS 4. At worst, it's nothing like the iOS 4 performance on the iPhone 3G. This video apparently shows the 3GS being faster than iPhone 4 (believable, actually, if you consider the quadrupling of pixels on the iPhone 4 from the 3GS):
http://techuncover.com/blog/tag/iphone-3gs-is-faster-on-ios-...