Assuming you know how to relocate stacks, you could implement efficient delimited continuations in Rust without the kernel's involvement, and use whatever it is you use for async/await scheduling today (so I'm not talking about some built-in runtime scheduler like Go's or Erlang's). But this would require changes in LLVM, and also a somewhat increased footprint (although the footprint could be addressed separately, also by changes in LLVM). The kernel work is only required if you want to suspend computations with non-Rust frames, which you can't do today, either.

That sounds like mioco/old M:N. That was slower than our current 1:1 threading.

I don't know what the old M:N model did, but a compiler can generate essentially the same code as it does for async/await without ever specifying "async" or "await." The reason for that is that the compiler already compiles any subroutine into a state machine (because that's what a subroutine is) in a form you need for suspension. But you do need to know what that form is, hence the required change in LLVM.

There is no inherent reason why it would be any slower or faster. It also does not imply any particular scheduling mechanism.