[fenomas]

> So in practice, in an attempt to produce high value but actionable feedback a hybrid approach, utilizing both micro/macro investigation yields the best results.

I disagree. JS is obviously not fast by its nature; the only reason it's fast sometimes is because modern JS engines do incredible optimizations behind the scenes. As such, for real-world performance, writing code that the engine understands how to optimize entirely dwarfs trivia like how Babel transpiled your default parameters. (Even the most microbenchmarked function will run dog-slow if the v8 optimizer bails out.) And if real-world performance is dominated by optimizability, then it follows that microbenchmarks are largely useless unless they happen to get optimized/deoptimized in the same way that your code does.

Incidentally for v8 at least, currently using any ES6 feature (even a single "const" that's never used) causes the optimizer to bail out. So any question of "ES6-induced bottlenecks" is beside the point - it doesn't matter how fast the ES6 feature is if its mere presence slows down everything else.