It'd be helpful to document in the performance section that if you're using Babble with the ES6 build then you should use loose mode

Stop using Set, because its not handled properly in loose mode

that's too bad. are you sure it's the use of Set rather than your spreading of it into an array? IE11 has Set support. you'd just need to forEach over it.

have you guys considered composition in lieu of classes (even without prototype or this). or something like:

function extend(parent, child, props) {
  child.__proto__ = parent;
  var proto = child.prototype = Object.create(parent.prototype);
  proto.constructor = child;
  for (var p in props)
    proto[p] = props[p];
  return child;
}

in uPlot i just stick to functions (most of them in a large instance closure) and have no issues with using Buble and the bundles are extremely compact. you have to avoid for..of and occasionally spreads, but those are really not on the usefulness level of e.g. async/await vs promises.

@leeoniya its the spread, but doing forEach with it removes the small speed gain it provides compared to simple object.

We've just about finished to conversion to classes, I doubt there are many volunteers to reverse that now. I testet buble and it created about the same code compared to babel loose, only 5kb bigger, because it did not alias this etc.