They don’t even realize they’re being held back by this. It’s an invisible friction until you notice it.
Easiest way to notice the effect is by switching to a new IDE where you don’t have the keyboard shortcuts in muscle memory yet. Or a language that’s easy but you have to think about the syntax because it’s new.
Or, in general, being able to use keyboard quickly. I've seen a lot of people who, when troubleshooting some issue in a SSH session, are v e r y s l o w in shell (quick sequence of little tasks like: type a command, find a particular command in your recent shell history, bring it back up, change something in it, navigate to a different directory, grep some log output).
If even the simplest of those tasks takes you a few seconds, you risk losing the train of thought.
I think of typing speed in coding the same way I think of cardio in boxing – it won’t win you the round, but the lack of it can easily lose you the round.
You want topping to feel seamless and reliable. Whatever you think should come out on the keyboard reliably and without friction. Any time you have to stop and think about typing or go back to check for typos, is time you’re losing focus and flow.
Take an extreme example – search-and-peck typing where every character takes 2 seconds. Can you really hold a complex programming context sharp in your mind while it takes a minute to type a few instructions? I sure couldn’t.
But you probably also can’t think fast enough to fill a 200wpm typing buffer.
Somewhere in between those extremes lies the sweet spot where your typing speed and your thinking speed match.