I agree with the JIT argument. JS can’t be as fast as on desktop if there’s no JIT compiler.
The threads I disagree with. You don’t need a bucket load of threads for an engine to be fast. It’s a can of worms so let’s not go there.
If an engine doesn’t use C++ then that shouldn’t affect performance, as it’s up to the vendor to optimise it properly.
Building on top of the IE engine is a lazy way to release a product on any Windows platform.