I think that https://pavelfatin.com/typometer/ gets close to checking the actual perceived quickness of a terminal. It measures the time from keyboard signal to pixels on the screen.

I would much prefer a terminal that still provided interactivity when text was scrolling in other parts than raw speed. Open up two panes in tmux or your favorite split screen with text scrolling in one and measure latency in the other pane with typometer and I will use whatever comes up to the top.

I think this works:

    START=`date +%S.%N` && find ~ && echo $START && date +%S.%N

Assuming the terminal programs don't 'announce' when they've finished rendering, the only way to this test properly would be a white-box approach, hooking into the code of the various terminal programs, right?

A good stopwatch.