I don't see your point. "Linux thread context switch time" is a meaningless metric, since Linux will switch thread context regardless of what you choose to run

I don't see your point. "Linux thread context switch time" is a meaningless metric, since Linux will switch thread context regardless of what you choose to run on your computer. Any "async" switches are additional overhead; you don't get to not have kernel preemption just because your Rust thread is now switching contexts "asyncly". There are benefits to having an additional user-mode scheduling mechanism inside your kernel thread, but saving CPU cycles isn't one of them.