It seems crazy to me that performance takes a backseat to syntactical sugar, which is most of what ES6 provides. There are a few new constructs, but in my mind when is it ever acceptable to have performance regressions to make the code look pretty?

And the fact that it isn't optional is alarming. If you use the new features, you should have to suffer performance penalty if there is one, and at least be able to make that design choice and trade-off between "cleaner" or "prettier" code and poor performance.

I don't think you can call something "complete" or green until the performance issue is solved.