[madeofpalk]

Last time I checked, fat arrow functions have a performance penalty because of all the optimisations that's made around scope in regular ES5 functions can't be used.

I believe ES6 fat arrow functions are slow for a similar reasons as why regular functions are slow when you use the magic arguments variable.

[venning]

Regarding the scope optimizations: that's an interesting point. I wonder if the different scoping of let/const also create a performance penalty versus var.

[madeofpalk]

This is true as well.