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Yes, it is expected perf regression compared to Python 3.5 and 2.7 when pass keyword arguments and single var-keyword argument (because 3.6 uses BUILD_MAP_UNPACK_WITH_CALL, while 2.7 and 3.5 don't need it for this case). In case of multiple var-keyword arguments (all versions need BUILD_MAP_UNPACK_WITH_CALL) even unpatched 3.6 is faster than 3.5.
$ ./python -m timeit -s "def f(**kw): pass" -s "b = {'b': 2}" -- "f(a=1, **b)"
Python 2.7: 100000 loops, best of 3: 2.21 usec per loop
Python 3.5: 100000 loops, best of 3: 4.31 usec per loop
Python 3.6 unpatched: 100000 loops, best of 3: 7.64 usec per loop
Python 3.6 patched: 100000 loops, best of 3: 3.14 usec per loop
$ ./python -m timeit -s "def f(**kw): pass" -s "a = {'a': 1}; b = {'b': 2}" -- "f(**a, **b)"
Python 3.5: 100000 loops, best of 3: 11.6 usec per loop
Python 3.6 unpatched: 100000 loops, best of 3: 6.93 usec per loop
Python 3.6 patched: 100000 loops, best of 3: 2.66 usec per loop
$ ./python -m timeit -s "def f(a=None, b=None): pass" -s "b = {'b': 2}" -- "f(a=1, **b)"
Python 2.7: 100000 loops, best of 3: 1.97 usec per loop
Python 3.5: 100000 loops, best of 3: 3.75 usec per loop
Python 3.6 unpatched: 100000 loops, best of 3: 7.27 usec per loop
Python 3.6 patched: 100000 loops, best of 3: 2.83 usec per loop
$ ./python -m timeit -s "def f(a=None, b=None): pass" -s "a = {'a': 1}; b = {'b': 2}" -- "f(**a, **b)"
Python 3.5: 100000 loops, best of 3: 10.6 usec per loop
Python 3.6 unpatched: 100000 loops, best of 3: 6.47 usec per loop
Python 3.6 patched: 100000 loops, best of 3: 2.31 usec per loop