iPhone 3GS users got huge performance degradation with iOS 4. Later Steve jobs promised that iPhone 3G user will see an improvement in iOS 4.1, But iOS 4.1 made things worse than before. After that Apple is confident over their promise on upcoming iOS 4.2 which improved a lots of things on iPhone 3G compared to iOS 4.
Tipb did a little performance test on iPhone 3G running on upcoming iOS 4.2 (developers release version) , of course got some positive results like decent job typing, smooth scrolling, pinching, and zooming.
In the following video you can see how iPhone 3G performed on iOS 4.2