> To some extent, all languages do this: there is almost always a tradeoff between performance and slower, safer implementations.

Well, probably except Rust, from which you get both performance and safety. Sometimes it's with a cost of ergonomics, though. Some people, including the Rust's creator himself, would trade performance for better ergonomics in the case of asynchronous programming. They prefer having a non-zero-cost green threads to the current state of async prog in Rust.