&gt; <i>ES6 performance sucks! Strong mode is a mode for ES6, you cannot use it without using various ES6 features. However, idiomatic ES6 code currently is substantially slower than ES5, across all browsers -- easily by 2x, often by 10x or more. Due to the sheer size of ES6, plus a number of unfortunate design choices, it will likely take years until implementations catch up with ES5 optimisations, and the hundreds of man years that went into those.</i><p>This has been my suspicion for a while.  Perhaps I&#x27;ve missed other comments to this effect, but I don&#x27;t think I&#x27;ve heard anyone else articulate it.