nothing makes me want to throw my computer through the window faster than typing and not having an instant response honestly. It doesn't matter that the brain cannot process the letter. Would you imagine handwriting where the shape of the letters you write is 1/4th of letter shape late ?
Even for typing, just comparing xterm at 60hz and 120hz on my monitor feels different when typing moderately fast (100 wpm according to https://typing-speed-test.aoeu.eu/?lang=en)
Not necessarily; it could be an electronic instrument.
I once tested a "e-piano -> midi -> PC -> headphones" chain. The latency was not really noticeable when I pressed only a single key, but actually playing was impossible. After I replaced a (software) component with something with lower latency, it was bearable. While I cannot precisely estimate the latency reduction, I would guess that it was less than 30ms.
We need to keep these numbers in context. 30ms delay would appear as an echo if you were playing an instrument and also listening to a 30ms delayed version of the same sound.
However, you're not feeling the latency. You're noticing the presence of two distinct signals. That's why UX input latency matters much more when you're doing something like dragging across the screen with your finger on the screen: You visually register the distance between your finger and the expected location.
However, when you're typing a memorized series of letters on a keyboard, you're not comparing the keypresses to the letters appearing on the screen. You already know what you're typing and what you expect to see on the screen. Visual stimulus latency is on the order of 250ms, so you wouldn't even begin to start processing types for an order of magnitude larger than the latency of these text editors, for example.
Most of us are operating with regular 60Hz laptops or monitors. End-to-end latency from physical key press to physical color change on your monitor might be as high as 50-60ms even with zero-latency software.