Discussion on Reddit led me to this official Python benchmark site. If you fiddle the settings you can get a comparison of how various Python 3 distributions compare to Python 2 on a bunch of benchmarks.
Bars below 1.0 mean Python 3 is faster than Python 2. The different colors are different point releases of Python 3. The broad picture here is that Python 3 is generally about as fast as Python 2 or a little better. The big slower in the middle is Python startup; that’s 2-3x slower now. No other obvious pattern to me.
I’d had in my head Python 3 was generally about 20% slower. Partly because it does Unicode properly now, partly because some of the switch from collections to iterables in the core tuple and list types added slowness. But that opinion is not born out by this data.
PS: this screenshot brought to you by Firefox’s awesome screenshot tool. Not sure if it’s new to everyone or just me, but it makes saving an image of a DOM chunk of a page very easy.