This is a case of "make it work right; then make it work fast". ES2015 performance will improve with time (we've already made 8-10x improvements on certain features) [0].

Strong mode work was stopped for a variety of nuanced reasons detailed here [1].

(Disclaimer: I'm a PM on the V8 team.)

[0]: https://twitter.com/addyosmani/status/699976753255751681 [1]: https://groups.google.com/forum/#!topic/strengthen-js/ojj3TD...

I can confirm this in my own projects as well.