I have not found mine to be slower after the upgrade. I have long found that my 3G will occasionally decide (especially when opening an app or the keyboard) to bog down for a few seconds at a time, but this has not changed with iOS 4.0.

Yes, iOS4 is definitely, absolutely, measurably slower than 3.1.x on any device. This is especially (painfully!) noticeable on the original iPhone and the iPhone 3G.

Newer devices have a much faster processor so the slowdown is not that evident on those (but still present).

I strongly discourage users of the original iPhone and the iPhone 3G to upgrade to iOS 4.0.

There is an updated 4.1 coming out which will be significantly better (faster!) on the older devices, but I suspect that they will not be faster than version 3.1.x is.