More than just these, even innocuous features such as `let` and `const` over `var` cause a substantial performance decrease in V8 (but not SpiderMonkey).
I do a lot of one-off demos, that I mostly write in ES6 these days. I don't have a demonstration of pure `let` vs `var`, but if you change Babel to JavaScript in this one (and click run), you'll see a fairly substantial performance decrease in Chrome for this demo (Firefox stays the same though): https://jsfiddle.net/oa1sckzu/ . I have others, and the results are largely the same for them, but I'll spare you.
I do a lot of one-off demos, that I mostly write in ES6 these days. I don't have a demonstration of pure `let` vs `var`, but if you change Babel to JavaScript in this one (and click run), you'll see a fairly substantial performance decrease in Chrome for this demo (Firefox stays the same though): https://jsfiddle.net/oa1sckzu/ . I have others, and the results are largely the same for them, but I'll spare you.