py3.6 string format vs concat

gistfile1.txt

> $ python3.6                                                                                                                                                                                   [±claymore-115 ✓▴]
Python 3.6.5rc1 (default, Mar 14 2018, 06:54:23) 
[GCC 7.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import timeit
>>> def d1(a, b):
...     return a + '_' + b
... 
>>> def d2(a, b):
...     return '%s_%s' % (a, b)
... 
>>> def d3(a, b):
...     return '{}_{}'.format(a, b)
... 
>>> timeit.timeit('d1("asd", "qwe")', setup='from __main__ import d1', number=1000000)
0.213849229999596
>>> timeit.timeit('d2("asd", "qwe")', setup='from __main__ import d2', number=1000000)
0.2879158160003499
>>> timeit.timeit('d3("asd", "qwe")', setup='from __main__ import d3', number=1000000)
0.3701504420000674
>>>