When last I checked, V8 simply triggers a deopt when it encounters any ES6 syntax at all - so just putting an unused &quot;let foo;&quot; in a function will kill its performance, regardless of whether any ES6 features are used.<p>Of course, it may be that some ES6 features will still be slow even after they&#x27;ve been optimized for, but right now, as a practical matter, I think ES6 is slow simply because the engines don&#x27;t yet optimize it.