> But if you have threads (green- or OS-level), you don’t need to do that. You can just suspend the entire thread and hop straight back to the OS or event loop without having to return from all of those functions.

Correct me if I'm wrong, but wasn't the lack of threads one of the biggest reasons why NodeJS originally outperformed most of its competitors?

Spinning up threads for each concurrent request was expensive, and (nonblocking) async code was by comparison ridiculously cheap, so the lack of overhead meant Node could just make everything async, instead of trying to decide up-front which tasks deserved which resources.

Granted, it's been over a decade since Node came out. Maybe thread overhead has gotten a lot better? But barring faulty memory, I definitely remember a number of people explaining to me back then that being single-threaded was the point.