Generally speaking, Python 3 is actually faster now. I am a bit surprised that no one else has commented on this. There was a talk about performance recently (https://www.youtube.com/watch?v=d65dCD3VH9Q). To sum it up, some parts are slower and other parts are faster, and the reasons depend on two questions:
1: Is it using a lot of small ints? Python 3 changed int from being small int to long int, and for work which deals with massive amount of ints this will result in a slow down. If for some reason you want to use a pure Python implementation of AES rather than using hardware acceleration (generally builtin to the CPU) or a C implementation, or the one built in the linux kernel, then you will hit the performance test that get the biggest negative difference between python 2.7 and master. Then there is numpy which uses C modules that can happy do things as small ints.
2: Bytes -> Unicode. Libraries that are Unicode unaware will run faster than libraries that are Unicode aware. The feature to understand that "ö" is a Swedish letter and not several characters does cost some CPU time. For parsing where per character manipulation is relevant (like say HTML), such parsing will be a bit slower in python 3.
Practically everything else is faster now in python 3.