The folks at AppleRumors Italia did a side-by-side video comparison of iOS 5 on both the iPhone 3GS and the iPhone 4. As you can see in the video, the two models perform similarly with only a small amount of lag apparent on the iPhone 3GS during certain tasks, like loading a complex web page.
The biggest difference is the lack of some features, like HDR and photo editing, on the older iPhone 3GS. Though this is only one video, the results are encouraging for those still rocking a third-generation iPhone.
