Folks at LifeHacker did some sort of speed test, comparing the performance of the iPhone 3G on iOS 4.0 and iOS 4.1 (expected to be released on Sept. 8).
To summarize the results, it appears that iOS 4.1 outperformed iOS 4.0 in Messaging, Maps, Photos, and Browsing. It could be possible that iOS 4.1 is faster than iOS 4.0 in all the apps. Anyway, take a look at this video.
So, what can you say about it?