Many benchmarks say JS-VMs are blazingly fast, at the same time others say they are battery-hogs. And Apple is even optimizing their CPU for it to run faster.

IMHO that is about JS-memory usage and GC, which never really get's proper attention.

You can have really fast algorithms, if you keep creating and forgetting millions of objects, which most JS-frameworks do, you will have lags and GC-pauses.