> And by cheap I mean cheaper than OS threads. The reactor pattern still adds a lot of CPU overhead to each I/O operation.

You need to pay for concurrency one way or another, there is always going to be some bookkeeping overhead. Whether you pay the price in your language's runtime (like Go) or in a library doesn't mean you're getting for free. However, it looks like the design of Rust's futures will let it reach very throughput.