`Map` polyfill (`get(key)`) is very slow

We recently starting using polyfill-library@3.34.0 to polyfill ES6 for (only) IE11 in Tableau. This caused a severe performance regression in our rendering code, which was traced to the Map polyfill. Removing Map and Set from our config resolved the issue – luckily our code only currently needs the native partial support for Map in IE11. (I don’t know if removing Set was also necessary, but since IE11 has partial support for both, I decided to err on the side of removing both.)

Unfortunately I have not investigated deeply enough to suggest a fix, but our profiling session indicated CreateMethodProperty(Map.prototype...) (23.1.3.6 Map.prototype.get (key)) was the culprit.

Here is our config, for reference:

const ie11_es6_opts = {
  uaString: 'Mozilla/5.0 (Windows NT 10.0; WOW64; Trident/7.0; rv:11.0) like Gecko',
  minify: false,
  excludes: [
    'Map',
    'Set',
  ],
  features: {
    'es6': { flags: ['gated'] }
  }
};

[JakeChampion]

@zloirock Good point! I missed that part when reading the spec, thanks for pointing it out.

@neenhouse, feel free to implement a more optimised implementation 😃

[JakeChampion]

@craigkovatch Yes please, if you could link it to this issue that would be great