This makes me think of Python 2 vs 3 benchmarks for some reason, although I'm sure I'm not remembering very well.

I assume there's plenty of scope for ES6 to improve and (hopefully?) surpass ES5 in performance in many, if not all, areas?

What would be causing the large slowdowns in ES6 here? Is this a case of having features that make JS nicer to code in, but giving up some performance? Or is this more down to immature compilers not yet optimising ES6 as substantially?