I disagree. I went from the original iPhone to the iPhone 4, expecting to be blown away by the speed difference. Guess what? Not much faster at all. Camera is still a closed shutter _forever_ while launching. Text messaging, maps, etc. slow to respond. Typing characters just as slow as ever - I've typed a whole word and then look at the screen to see the letters i n d i v i d u a l l y appear. I still love my iPhone4, but I can't imagine how you could have seen a difference from the 3G to the 3GS when Original to 4 was minimal improvement. Don't get too excited for the 5.
I don't know if it's your phone or your eyes that are messed up... but my 3Gs was WAAAY faster than my orignal iPhone... and my iPhone 4 is WAAAY faster than my 3Gs was... Maybe some people are just hard to please?