I don't think it's instantaneous, though it would be fast. Whirring metal blades can only spin so fast, and a human-size grinder would be much larger than a chick-sized one, so it wouldn't be able to spin as fast. Also, assuming you drop the human into the blades feet-first, just using normal gravity, there's going to be some amount of time between the feet getting ground up and the head. Whether this is perceptible to the doomed, I guess would require some calculations involving nerve conduction speed.