To be honest, the last great speed improvement was going from the 3G to a 3GS -- even in 2008, the 3G at launch felt sluggish then!
The iPhone 4 does feel snappier, but it's not really dramatic or jaw-dropping. The main improvement is the higher res. screen. Lately, my iPhone 4's been sluggish in between menus and switching apps. I am really motivated to just do yet another restore. It almost feels as if Apple put something in the code that forces the phone itself to slow down as time progresses in order to look forward to new iPhone hardware in the future. Conspiracy much on my part? Hehe.
I'm just expressing my opinions on the iP4 and how everyone seems to praise the speed improvement. It's nice, yeah, but it could really, really use more processing power. Waiting more than 4 seconds for the stock camera app to open is still unacceptable.