I think it is more of a placebo effect than anything else. I felt iOS 4 was fast when I first installed it on my 3G. You all know how that turned out! Very soon I had to throw my 3G in the trash because it became borderline unusable with its lag. So I am going to reserve judgement at present. All I am going to say is iOS 5 runs smooth on my iPhone 4 for now.
Mine too, especially when browsing through my over 3500 pictures. I think part of it is just the restore process that was used, which wipes all the data and writes it again. I think that puts everything in order on the flash so it isn't randomly reading from all over when doing things like browsing pictures.Mine seems slightly zippier.