I think it's a well known "fact" that 3 is slower. Probably a bunch is due to proper unicode handling?
Honestly, for MMA, it really doesn't matter. I compile 1100 songs in 2 minutes (just under 3 minutes in py3). So, that's about 0.1sec (one tenth of one second!) per song. And, I'm pretty sure that most of the time is spent in starting up python, not running the actual code. Of course, this will vary a lot depending on your processor (I'm running an Intel(R) Core(TM) i5-4570 CPU @ 3.20GHz which was pretty much top of the line a few years ago.
But, really, I have no idea. I just know of the purposes of MMA python is plenty fast