I just watched a b&w French documentary film from the '40s, called Le Sange des Betes, which gives a very clear view of the killing and dismembering of horses, cows, calves and sheep.You can't look at the abbatoir, food safety regs mean they won't let you in.
Back then, it appears that the smaller animals at least were killed by slicing through their throats and at the same time, the rest of whatever's in the neck - no stunning. But it's evident that loss of consciousness is almost instantaneous, due in main to a complete and sudden drop in blood pressure, I imagine. Given a skilled and experienced worker, I can't really see any 'benefit' in stunning first... The bigger animals seemed to have got 'the luxury' of a captive bolt or pole axe, due I imagine to their size making a swift decapitation near impossible.
As the director points out, "If it were in colour, it'd be repulsive... the sensation people get would be physical one."