Hanging from a pull-up bar isn't easy, is it?

Since I sit in a chair for long periods, I worry about my posture slouching and getting turtle neck,
so I bought a doorway pull-up bar and hang from it whenever I have time.
But hanging for just 20 or 30 seconds is definitely not easy! Wow.
It actually feels easier to just do 3 or 4 proper pull-ups. Wow.
Doing it for over a minute is seriously intense. I can barely do it if I really set my mind to it and squeeze out every bit of strength; usually, my strength gives out once I pass 20 seconds. Wow.