You're the one who said "you literally can't tell the difference". I agree to a point. It seems plausible that they were experiencing some other effect such as hitching or slowdown, rather than just a constant 100ms delay (which again isn't supposed to happen).
On the other hand, I just thought of one way that even a small fixed amount of latency can be a material obstacle. Personally, I type fast but make lots of typos, and I don't use autocorrect. So I need to react to incorrect text appearing on screen, backspace, and retype. The slower I react, the more text I have to delete (which means not just more keypresses but also more mental overhead figuring out what I need to retype). For this purpose, I am bound by the human reaction time, but editor latency is added on top of that. The sooner the text appears, the sooner my 'reaction timer' can start, all the way down to 0 latency. [Edit: And 100ms of latency can make a meaningful difference here. I just did a quick typing speed test and measured 148 WPM which is around 12 characters per second, so 100ms is one extra character, or a bit more.]
Also, latency might affect productivity just by being annoying and distracting. YMMV on whether this is a legitimate complaint or whether you should just get used to it. But personally I'm glad I don't have to get used to it, and can instead just use editors with low latency.
On the other hand, I just thought of one way that even a small fixed amount of latency can be a material obstacle. Personally, I type fast but make lots of typos, and I don't use autocorrect. So I need to react to incorrect text appearing on screen, backspace, and retype. The slower I react, the more text I have to delete (which means not just more keypresses but also more mental overhead figuring out what I need to retype). For this purpose, I am bound by the human reaction time, but editor latency is added on top of that. The sooner the text appears, the sooner my 'reaction timer' can start, all the way down to 0 latency. [Edit: And 100ms of latency can make a meaningful difference here. I just did a quick typing speed test and measured 148 WPM which is around 12 characters per second, so 100ms is one extra character, or a bit more.]
Also, latency might affect productivity just by being annoying and distracting. YMMV on whether this is a legitimate complaint or whether you should just get used to it. But personally I'm glad I don't have to get used to it, and can instead just use editors with low latency.