"We want you to help us! [...] Ideas for real ES2015 performance test suite."

* Take a language that compiles to JS, with the option to target either ES5.1 features or ES2015 features. For example, Scala.js.

* Take a macro-benchmark suite, or an application with measurable performance metrics, written in that language. For example, https://github.com/sjrd/scalajs-benchmarks

* Run the benchmarks when targeting ES5.1 (default in Scala.js) versus ES2015 (with `scalaJSOutputMode := OutputMode.ECMAScript6`)

* Enjoy the slowdown :-p

I did that 1.5 years ago when I was developing Strong Mode support for Scala.js (now defunct). Results were awful at the time: https://groups.google.com/d/msg/strengthen-js/Bfa_HPhVbiY/yM... They probably have improved since then; I should redo the benchmarks one of these days.