Hi Alex,
I did some performance testing with these operations on Node.js on my MacBook. I did not release the results because they should not be seen as a professional benchmark of low level operations (because they are definitely not). Insertions turned out to be a bit slower on Maps but lookups are way faster. However, in many cases you might want to store keys that are not strings. The conversion into strings destroyed any speed advantage of a plain object in my tests runs.