They're killed as humanely as it's going to get. A bolt through the head. The bolt going into their head arrives faster than the central nervous system can send the signal that "ow, that hurt". Everything after that is the death rattle.
Now "religious" killing, like in Islam and Judaism, where they just hang the animal upside down, cut it's throat and let it bleed out... that's a different story and takes the animal much longer to die.
In small scale abbotars you may be right, but most of the industrial ones are awful before they even get to the bolting (if it works properly- which it doesn't on all).
Just an FYI, most/all religiously slaughtered animals are stunned in the UK before dispatch, just like non religiously slaughtered animals.