Part of the work Igalia did in 2016 was to improve compatibility and performance of native Promise. It's a constantly moving target. If you single out v8, they're constantly improving the JIT, evolving the architecture, while individual features (e.g. generators, Promise) continually get optimized on top of those underlying engine changes. Lots of people talk about performance in absolute terms, but what is really needed is a proper performance test suite for individual features and a kangax style[1] browser/engine version perf chart (edit: like the link you posted, duh!) to track it over time similar to the more general AWFY[2]. I guess the main issue is that the test suite needs to be more accepted by the browser teams as a de-facto standard?
[1]: http://kangax.github.io/compat-table/es6/
[2]: https://arewefastyet.com/