As previously reported, a number of iPhone 3G users have experienced a significant degradation in performance after the iOS 4.0 update. iPhone 3GS users generally report across-the-board performance improvements with the new update, but original iPhone 3G users have reported slow app launching, delayed reactions to taps, and generally poor interface responsiveness.
Anecdotal evidence from a number of users indicates that the forthcoming iOS 4.1 release, which just entered a private beta (accessible only to iPhone developer program members), will include significant performance improvements for the iPhone 3G.
In the meantime, here are a number of fixes for iPhone 3G slowness after the iOS 4.0 update.