a text editor (especially for code) is a good example of a case where both matter. it doesn't take much latency to make typing a very frustrating experience. the common case (editing small files) needs to be very fast. but we also need to gracefully handle very large files that involve nontrivial processing (10kloc+ c++ files do unfortunately exist). I gave up on atom several years ago when it slowed to a crawl opening a 4kloc c file. vs code can handle many multiples of that without breaking a sweat, so it's my current first choice.

Not in the case of a text editor. But feel free to hand-code your editor in assembly.