Might be a daft statement but I thought the focus-speed was more down to the lens motor than the body.
There is some truth to that - the weakest link, only as fast as the slowest ship, etc.
As was said by kayemod, the body processes the info and commands the lens accordingly. The faster, but more importantly, the more certain the body is of where to focus, the better. The lens obeys as quickly as it can.
May modern lenses are no longer driven by gears but by electomagnetic motors that react very, very quickly. What I found was that my D300 dithered on the decisions and the lens hunted. It hunted quickly, mind you, but still hunted.
Also, some solid colours are very hard for the focus systems to "see" but that facility is getting better, too, with more modern beasts.