[s3th]

A big difference between ES5 and ES6/7 is that the former passes through our old, highly-tuned optimizing engine Crankshaft [0] and the latter passes through our new optimizing engine TurboFan [1]. The team's expertise optimizing Crankshaft can certainly be leveraged in TurboFan, but it's not an apples-to-apples comparison. In the future, TurboFan will handle ES5 code as well.

tl;dr we're changing engines mid-flight and that has as much to do with performance differences as the ES6 features themselves.

[0]: http://blog.chromium.org/2010/12/new-crankshaft-for-v8.html [1]: http://v8project.blogspot.com/2015/07/digging-into-turbofan-...