Features aside, should I expect a performance increase in general or the reverse?
Sorry, but the Wikipedia article for iOS 5 indicates that iOS 5 does not support the iPhone 3G.
Everything I have read on the subject indicates a high probability of reduced performance when upgrading a 3G from iOS 3 to iOS 4. iOS 5 is not supported on the 3G. There were also some crashing issues with iOS 3.1.3 and later. I've kept my 3G at 3.1.2, however more and more apps are dropping support for iOS 3.
When upgrading my iPhone 3G to iOS4 I had a dramatic loss of performance. iOS5 is not supported by iPhone 3G.