That's interesting. Every deer I have shot straight through the vitals with a .50 Powerbelt has dropped at the shot. The first one fell before the smoke cleared and I couldn't figure out where he went. Muzzle loaders aside, I do agree that speed kills (with a proper bullet).My experience tells me that Roy Weatherby was right from the beginning. "Velocity kills", with a properly expanded bullet, of course. The higher the velocity, the faster the incapacitation, all other variables being equal.
I think the slowest kills that I've experienced have been from my muzzleloader. The bullets are certainly wide enough and heavy enough but they lack speed. High velocity impacts seem to disrupt the CNS even if the bullet doesn't impact in a CNS area.
Big & slow certainly gets the job done but doesn't seem to incapacitate as quickly.
Just my observations.