I’ll intellectually/emotionally/physically hard as answers. For me its either 12 hours straight “punching tubes” on a very large scotch marine firetube boiler at the beginning of my career or Easter around a decade ago when I was working with troubled teens and had to engage in 5 separate protective holds in one 16 hour double shift. The former was all physical and the latter was a combination of emotional and physical.
Simultaneous interpretation. My first time out I did 20-30 minutes and had to lie down immediately afterwards. You’d think that just listening to someone else talk and then just repeating them in another language would be easy, but you have to buffer quite a bit to get the interpretation right, and then talking and listening at the same time is also pretty hard.
Sounds incredibly mentally draining.