The emterpreter is really an interpreter, but as you guessed, you can direct the compiler to compile parts of your code to ASM.js so that you can benefit from native-like performance for CPU-intensive parts of your app.

Yield is relatively new, part of ES6, and I think is tied into async/await. It might be possible to use it in combination with something like Babel, but I didn't go down that route. I think the way forward is SharedArrayBuffers and their blocking wait()s on futexes.