I am curious how hn would construct a program which tried to minimize the latency of "simplest" test possible. Lets say -- press the space bar and the pixel at top right of screen turns green.

I used to use nasm on a dos machine - where it was possible to write a rather straightforward assembly program that would take a keypress and update a pixel -- but I am not sure how abstract/layered graphics stacks are on operating systems these days -- IE -- what is the simplest/low latency method to tell a computer to flip an pixel? SDL?