"""
./python.exe -m timeit -r11 -s 'import datetime' -s 'a = datetime.datetime.now().isocalendar()'
50000000 loops, best of 11: 8.72 nsec per loop

Let me know if the benchmark code is not appropriate
"""

Hum wait, this benchmark measures the performance of the pass opcode... That's why it's so fast :-) -s 'a = datetime.datetime.now().isocalendar()' is run exactly once for the whole benchmark, not at each benchmark iteration...

I suggest you this microbenchmark instead:

./python.exe -m pyperf timeit -s 'import datetime; dt = datetime.datetime.now()' 'dt.isocalendar()'

For example, on my Fedora 30 with Python 3.7.4, I get:

Mean +- std dev: 108 ns +- 4 ns