If you know your phone is aged expect new software to show this aging.
Two years is not aging, man! If Snow Leopard would've made my MacBook crawl without a possibility to downgrade, I would have personally sued Apple.
I bought the 3G as soon as it was available, but someone could've bought it new a few months ago.
If a Blackberry/Nokia/Samsung gets a software update after two years, are you saying one shouldn't expect it to work properly?
The problem is not that iOS4 has so many features that 3G can't handle them. The only visible possibly performance-hungry feature is folders, and I had used (3rd party) folders on my iPhone for a year! I even had multitasking and a background image on iOS3. The background image on iOS4 made the performance completely unusable, but I actually had to install jailbreaked multitasking just to be able to shut the background processes.
The problem is that either Apple released some sloppy coding, or that they are trying to make people want newer devices. There are several members at this forum who bought an iPhone4 because iOS4.0 didn't work properly on their 3G, so it is working. I felt the urge too, but decided to skip this one.
Though I must say, in the video the iOS4.0 is not actually half bad. Doesn't relate to the severity of the issue I had before jailbreaking.