Anything that you need to do a lot of, in a second, suffers for wasted microseconds.

If your input processing and font rendering pathways waste microseconds (and they do) it makes a delay in characters you type showing up on the screen, and a corresponding, measurable reduction in your editing throughput.

When our machines were a thousand times slower, characters showed up on the screen in substantially less time than they do today. The difference is mainly a result of cumulative wasted microseconds, in so many places that nobody can afford to gather them up.