You would think if it was so painful the rat would be struggling or squeaking. It takes 10 seconds for my rats, and they just fall over. I think it's much less painful than whacking its head, and hoping it works the first time.
Besides, rats have a natural painkiller built into their brains. Much like our adrenaline. It's not any more painful than the bp doing his job. I let my bp do it until they upgrade to a large small rat.
And by suffocation, you don't have to worry about getting blood anywhere. The snake gets it all.