This was a response to the leaked video of a poorly done shechita from a kosher slaughterhouse in Iowa a few months ago.
"So I personally am not sure that there is such a thing as a cow going unconscious instantly after the initial act."
There is no reason to think it would happen instantly, but if done correctly it does happen quickly. If both carotid arteries are cut, sheep will go unconscious between 2 and 15 seconds (Nangeroni and Kennett, 1963; Gregory and Wotton, 1984; Blackmore, 1984). Cows can
have prolonged periods of consciousness, especially if they are excited or if the cut is made slowly. But a quick cut, high near the jaw on a calm animal will cause very little pain and consciousness can be lost between 5 and 60 seconds.
There is no reason why shechita can't be done in the most humane way even though an animal can be kosher after a slow cut while being excited and in pain from handling. Basically, a kosher slaughter can be very humane - but shechita does not automatically equate into humane treatment.