I had the same problem with 3G and iOS 4.0, but after a hard reboot it became fast again (Hold the sleep button for some seconds, and push the slider). It took

I had the same problem with 3G and iOS 4.0, but after a hard reboot it became fast again (Hold the sleep button for some seconds, and push the slider). It took me almost a week to figure it out (during which the phone was next to useless), but now it seems as fast as 3.x.