TLDR, use Babel in "loose" mode and you'll be fine for pretty much all the ES6 syntax features (by which I exclude Maps, Sets, and generators). Most of the fea

TLDR, use Babel in "loose" mode and you'll be fine for pretty much all the ES6 syntax features (by which I exclude Maps, Sets, and generators). Most of the features listed are zero-overhead when transpiled this way. As usual, the native implementations are much slower for some reason (probably because they aren't optimized yet). Microbenchmarks and relative speeds are not super useful. I'm a performance nut and I love optimizing code -- back in the day I wrote a syntax-highlighting editor that was snappy in IE6's crappy JScript engine -- but I'm not going to worry that some syntax feature is 3x or even 10x slower than assigning to a local variable (which is what, one CPU instruction?). If you're that concerned, you should be avoiding object allocations ({}) like the plague, and that is just madness except in really performance-critical sections of code (and games).