[s3th]

You can see comparisons between ES5 and ES2015 feature equivalents, as well as comparisons with older versions at https://kpdecker.github.io/six-speed/.

It's important to remember that implementation of ES2015 is merely the first pass; optimization comes soon after and will rapidly improve performance. General, absolute numbers (e.g. "ES2015 is X% slower than ES5") are usually either nonspecific or soon out-of-date. The best advice is to revise your performance assumptions frequently based on real code.

r.e. ES5 side-effects: ES2015 adds language features and changes semantics of existing features in ways that necessarily change the execution path of code (including ES5 code). When V8 first tried implemented proper (ES2015) RegExp subclassing, for example, it regressed existing (ES5) RegExp performance by ~70%, but a subsequent optimization reduced the regression to ~9%. This is an unfortunate consequence of a rapidly evolving spec, but these cases are exceptional and will get better over time.

(Disclaimer: I'm a PM on the V8 team.)