I meant the UI performance. In terms of raw performance, the iPhone 4 still does things much faster than the 3GS. What I'm having problems is with the UI performance. The iPhone 4 seems pretty choppy in the UI animations and fluidity compared to the 3GS.
Sorry, they're not smoother on the 3GS vs 4. I've stood with my old iPhone 4 side by side with a 3GS in the Apple store in Glasgow when I was considering an iPhone for my wife, and the performance of the 3GS in the UI kit department was abysmal. It seemed slower, like turning on FakeClockUp and setting it to one setting BELOW the norm. I ended up getting my wife the 4, even though she didn't need the features of the 4, but I wasn't putting her through the hell of using a phone THAT slow.
I found the best way to make an iPhone 4 as smooth as a 4S in the UI kit was to only use 3 rows of icons (3 rows + 1 app on the 4th row was ok) and restrict folders to 4 apps per folder.
But the 3GS isn't smoother than the 4 on iOS 5, and it sure as hell won't be the case with iOS 6. I actually have no idea why the 3GS is being given iOS 6 given that the feature set is further reduced. Just get the new stock wallpaper using a Google search and away you go, because that's as much as you get that's worthwhile.
Thankfully the 3GS will be killed off on Wednesday.