Anecdotal evidence suggests that the third beta of iOS 4.1, which was recently released to developers but is not yet publicly available, significantly enhances the performance of the iPhone 3G, which was slowed significantly for many users after the initial update to iOS 4.0.
Several developers who have updated to iOS 4.1 beta 3 report much better responsiveness from the iPhone 3G.
Some iPhone 3G users who updated to iOS 4.0 (and subsequently iOS 4.0.1) began to experience slow app launching, delayed reactions to taps, and generally poor interface responsiveness.
We previously published a series of fixes for iPhone 3G + iOS 4 slowness, with which many users have experienced success.