While formal benchmarking is tricky on the iPhone, my initial
impression of iOS 4 on my iPhone 3GS is that it’s quite snappy
— more so than version 3.1.3. The OS as a whole seems more
responsive, but there are a couple of places where speed
improvements are pronounced: in general, the Camera app running on
an iPhone 3GS seems far zippier now than previously. Pictures get
taken almost instantaneously, and I didn’t notice any of the
sluggishness that has occasionally plagued iPhone 3.0.
I’ve been running the betas (and, since WWDC, the GM release) on my 3GS for weeks, and I agree. iOS 4 feels faster than 3.x on the same hardware.