I was suspicious that there might be some artifacts of terminal updating that are amplifying the effect in that test, so I whipped up a little graphical test:

https://jsfiddle.net/zs8bncxj/1/

I can still get it right pretty much 100% of the time, but the difference does feel a lot more subtle than what I was seeing in iTerm.

(If you change the delay, you need to hit randomize to make it take.)

Edit: Also we should keep in mind that we're not testing the latency we set in either test. We're testing that delay plus the delay from I/O. So 100ms might be indistinguishable. But by the time we pile everything on, we're getting closer to 200.

The result is the same as far as UI design is concerned; don't assume that you can get away with 100ms; most of that leeway has already been wasted by the OS and hardware.