At the level of a programming language, the win from async isn't one of performance (at any given time you're operating at most one process per core, and you'd

At the level of a programming language, the win from async isn't one of performance (at any given time you're operating at most one process per core, and you'd generally get better performance if it were always the same process for each core doing exactly the right thing at the right time, including choosing when to interact with your asynchronous network card), but rather one of developer ergonomics; it can be a lot easier to interact with async things (those network events) by making everything that calls into it async as well and relying on some framework to turn your async code into something that actually matches the machine's execution model.