With JS' async/await system basically running on creating temporary closures, I don't think things will change all that much to be honest.

Furthermore, I don't see why engines should police what is or isn't acceptable performance. Using functional interfaces (map/forEach/etc.) is slower than using for loops in most cases, but that didn't stop them from implementing those interfaces either.

I don't think there's that much of a performance impact when comparing

    const x = fun1(abc);
    const y = fun2(x);
    const z = fun3(y);
    fun4(z);

and

    abc |> fun1 |> fun2 |> fun3 |> fun4

especially when you end up writing code like

    fun1(abc).then( (x) => fun2(x)).then( (y) => fun3(y)).then((z) => fun4(z))

when using existing language features.