Before we go into the details of the various improvements, we should first consider why performance of ES2015+ features matter despite the widespread usage of Babel in modern web development:
- First of all there are new ES2015 features that are only polyfilled on demand, for example the Object.assign builtin. When Babel transpiles object spread properties (which are heavily used by many React and Redux applications), it relies on Object.assign instead of an ES5 equivalent if the VM supports it.
- Polyfilling ES2015 features typically increases code size, which contributes significantly to the current web performance crisis, especially on mobile devices common in emerging markets. So the cost of just delivering, parsing and compiling the code can be fairly high, even before you get to the actual execution cost.