This is probably the first time I heard of such a speedup when going from 2.7 to...

dr_zoidberg on July 7, 2015 | parent | context | favorite | on: What’s New in Python 3.5

This is probably the first time I heard of such a speedup when going from 2.7 to 3.x, and the fact that you can't account for it makes it worse because we can't evaluate what's happening and if such speedup could be achievable on other projects :(