[fenomas]

When last I checked, V8 simply triggers a deopt when it encounters any ES6 syntax at all - so just putting an unused "let foo;" in a function will kill its performance, regardless of whether any ES6 features are used.

Of course, it may be that some ES6 features will still be slow even after they've been optimized for, but right now, as a practical matter, I think ES6 is slow simply because the engines don't yet optimize it.