Fast zombies just don't seem logical, if that is even applicable to zombies in general. Most of fine motor control is not in the reptilian part of our brain.
To which I must ask a TWD question. Say I shoot a zombie in the brain they die, but why? TS-19 shows that the brain itself is not used for anything. You would think you would have to destroy the reptilian part of the brain because as we have seen severing the head just makes the head 'talk' to you and attempt to 'get' to you.