That's not sufficient, though. You want the 0.1% case where you press undo a couple of times and big edit operations get reversed to be smooth. You have to hit your frame target when a lot is happening, the individual key press case is easy.

It's just like a video game. A consistent 60fps is much better than an average frame rate of 120fps that drops to 15fps when the shooting starts and things get blown up. You spend all the time optimizing for the worst case where all your caches get invalidated at the same time.

The other latency that matters is how long it takes to draw each frame when scrolling through the code, which is usually most if not all of the framebuffer.