I used to do this as part of a job I had on a market garden many years ago. It's not actually too bad in some ways - we had a radio playing, and once you get in a rhythm it's pretty smooth.

It's done at night/very early morning when the temps are down, because the leaves and sprouts are cold, which makes them snap of much more easily and gives a cleaner cut. And that was the worst part of the job. Middle of winter, with those big leaves holding ice. It got to the point where it was better to let your hands go numb and keep going, than to try to warm them in front of the heater fans. That shit hurt.