They didn't just slap an S on the back and call it a day, you know. Apple has improved the 3D performance on the 3G S by including a more powerful graphics processor. According to John Casasanta, the speed increase is up to four times that of the 3G. But don't take his word or these OpenGL ES speed tests as proof -- check out the video and see for yourself.
On a related note, we hear that Apple has done "even more" than we can publicly disclose around speed improvements on the device. That could mean there are further software updates that may make the 3G S even faster. That's the kind of thing Apple loves to do, isn't it? Just look at the Snow Leopard marketing around it being lighter and more efficient. I've usually had (often temporary) system speed increases with updates as well, so here's hoping there's some unused horsepower under the hood just waiting to be unleashed.