I doubt if sleeping on your back is enough to cause a significant change in the speed of nerve impulses through your spine.
There is a theory, however, that the reason that sleep paralysis most commonly happens when you are on your back is that it is related to
tonic immobility. This is an involuntary response that a prey animal has when a predator places it on its back - it automatically becomes paralyzed so the predator may think it is dead and leave it alone.