Ah, but you completely missed my point.
I already said I understand your point.
BUT, what if a pitcher is large, has long limbs, and tendon attachments relatively close to the joints (worse leverage).
If he moves quickly he will never be able to coordinate everything. And well if he doesn't have as much muscular coordination because of his size he won't even be able to activate all of his muscles in a short amount of time.
If he slows down he can use his massive levers to generate force. And he can fully activate more motor units in his body.
I assure you more motor units > moving hips forward faster.
The longer the levers, the slower you can move and generate the same amount of force.
Sure Randy Johnson starts moving forward early and he's big, that's fine. However the prototypical fast mover on the mound is SMALL.
Lincecum for instance.
We had a high school kid here hitting 92. He only weighs 175lbs and that is what part of people marvel at. He moved extremely fast. People also predict he will get hurt soon because he moves fast and violently.