I'm sure it is a thousand times slower than i++, but it should be the same code on each platform.

But you made me think: indeed, the underlying call could be slower from JS whereas from pypy/python{2,3} it's fast (or vice versa). So I just tested a version where it checks how long ten million iterations take (n=3). Code here: https://hastebin.com/uyuhecabek.py

    browser/pypy   5.25 seconds
    python 3.6.6   2.16 seconds
    python 3.7.0   2.08 seconds
    python 2.7.15  1.42 seconds
    pypy   6.0.0   0.01 seconds

Since there is no syscall to be made, pypy can do this in compiled code and is blazingly fast (I bet C/assembly isn't much faster there), and the rest seems comparable to the previous estimate: the browser version is 2.25× slower compared to python3 (now 2.43×) and 3.33× slower compared to python2 (now 3.70×).