In chrome I can get 16.7 million key/value pairs with Map vs. 11.1 million with a regular object. Almost exactly 50% more pairs with a Map. They both take up about 2GB of memory before they crash, and so I think may be to do with memory limiting by chrome (Edit: Yep, try filling 2 Maps and you only get to 8.3 million pairs each before it crashes). You can test it yourself with this code (run them separately and not at the same time, obviously):
Только что в chrome отработал цикл со ста миллионами ключей obj, Map в такой же ситуации вылетел. Не могу воспроизвести эту информацию, наверное устаревшая.