Nethack also runs pretty well on old GPUs. I consistently get >60fps even on integrated graphics.
But you had me going for a second. Nethack is also turn-based, so there's no real-time element at all. If you come across a difficult turn, its a common "strategy" to walk away from the computer, relax a bit, and then look at the screen with a different mindset. Maybe you can think of a better solution if you give yourself time.
(Although I assume nethack is more clever than just "re-send the whole screen for every change".)
No one said anything about the data changing ...
(and yeah, I believe it only sent data for the delta.)