There seems to be some idea that killing is ok as long as it is instant and the animal feels ok right up until the moment they feel nothing. Surely this cannot be all that unethical.
If animal slaughter is so painless and quick, would anyone reading this volunteer to be "painlessly slaughtered"? Would you pay to have a perfectly healthy loved one "painlessly slaughtered"?
If not, then one must ask themselves why murder is wrong. In any other context, killing being wrong has nothing to do with the suffering that one endures before death. For instance, is it OK to be a serial killer that only kills people who are living a good life under the pretense that one did it instantly? Of course not. Then the only way to apply this to slaughtering animals is some sort of special pleading.
Animals killed by captive bolt pistols often need "re-stunned", and one can only imagine the number of calves that make it to the throat-slitting portion still alive but just too weakened to struggle enough that it makes the job slower. Because slaughterhouses are judged by their speed and volume in the end, animal welfare leading up to slaughter is not a typical consideration, even if cameras are rolling.