What percentage of users actually, really care how marginally more efficient their web browser is these days?

Given that most computers I see have a billion tabs open, I'd guess most of them.

I'm not saying that many users will care about their browser's performance. I'm just saying that that is a potential reason to use Edge.

It's the same Chromium, it can't be more resource-efficient. It did have some tricks, but “energy saver” and “memory saver” modes have been added to Chromium.

It can be more efficent on Windows where Edge is first class passenger similar to how Safari is more efficient on MacOS. Try looking at battery life. You can easily squeeze more juice on Edge than on FF/Chrome.