[jerf]

You will absolutely get "more" performance out of async. I'm not sure I could call it much more. It's hard to get an exact number because there isn't exactly a whole lot of pairs of "async" vs "greenthreaded" options out there, but I'd guess you're looking at 20%-30% tops. For most people, and even most people writing async code, this is irrelevant. They are never going to write code that absolutely needs that last 20-30% and that alone is the difference between the problem being solved and not solved.

It certainly isn't like you use a green thread model and you unconditionally throw away a 5x performance factor or something.

There are absolutely cases where that does matter. To name just one, a game engine would not want to throw away that level of performance out of the box. (That's the game engine user's job, to "spend" the quality of the game engine on their task.) But I think there's a lot more programmers who have, without analysis, assumed they're in that class and made a lot of decisions based on that, when in fact they are plural orders of magnitude away from it. To pick a number out thin air, 4 full CPU cores running Rust code that someone has at least glanced at and spent a bit of time optimizing is a loooooot of power.

(The closest current comparison is Rust vs. Go, but Rust works much harder at compile-time optimization and doesn't have GC, and I expect those two things account for the majority of the delta between them, with Go being greenthreaded being non-trivial, but in the clear minority. Stay tuned for Java with Project Loom versus Rust, which has its own rather major differences but will at least be another relevant data point.)