Interesting! ''.join() has the advantage on longer strings though:

$ python -mtimeit "'aaaaaaaaaaaaaaa' + 'bbbbbbbbbbbbbbb' + 'ccccccccccccccc' + 'ddddddddddddddd'"

1000000 loops, best of 3: 0.224 usec per loop

$ python -mtimeit "''.join(('aaaaaaaaaaaaaaa','bbbbbbbbbbbbbbb','ccccccccccccccc','ddddddddddddddd'))"

10000000 loops, best of 3: 0.201 usec per loop

That's definitely interesting.

Always worth checking. Although the relative merits change when doing many joins to build up a single long string.

This recommendation is to go with .join() by default:

In performance sensitive parts of the library, the ''.join() form should be used instead. This will ensure that concatenation occurs in linear time across various implementations. — http://www.python.org/dev/peps/pep-0008/

despite the fact it might not be better in CPython.

It's not terribly surprising. Things that scale faster tend to have higher setup costs (cf sorting algorithms; insertion sort is the fastest for relatively small n).

I believe the Python runtime has special-case handling for concatenating string-literals, and for concatenating string whose reference count is exactly 1. String concatenation isn't wholly defanged, though.

You're also constructing the array in the second test.

For a fair test, add elements of an array vs. joining them.