The 3G to 3GS was only a minor boost in speed -- the 4 to 4S was a more significant boost.
While technically that might be true, in practice you felt the speed increase more. The 3G felt like it lagged with pretty much everything you did and the 3GS eliminated the perceived lag everywhere. The 4 never felt like it lagged so the 4S didn't have the perceived speed increase like the 3GS did. Not to mention the 3GS had triple the usable RAM.