iOS4.1 promises to increase the poor performance that iPhone 3G owners felt when they updated to iOS4. Website Lifehacker ran some tests to see which Operating system works better. Text messaging is definitely improved with 4.1, by a longshot.
Maps seem to load much faster with 4.1 as well. Photos and Mobile Safari are also improved. The video is worth a look, and while it may not be an official speed test, the side by side comparison shows some great results for the new operating system.