I got following result:

$ ./python.patched -m perf timeit --compare-to=./python.default --duplicate=100 -- '{}'
python.default: ..................... 203 ns +- 5 ns
python.patched: ..................... 97.1 ns +- 0.7 ns

Mean +- std dev: [python.default] 203 ns +- 5 ns -> [python.patched] 97.1 ns +- 0.7 ns: 2.09x faster (-52%)

$ ./python.patched -m perf timeit --compare-to=./python.default --duplicate=100 -- 'x={}; x[1]=1'
python.default: ..................... 494 ns +- 5 ns
python.patched: ..................... 592 ns +- 7 ns

Mean +- std dev: [python.default] 494 ns +- 5 ns -> [python.patched] 592 ns +- 7 ns: 1.20x slower (+20%)

Seems something is wrong with resizing an empty dict. It shouldn't take such much time.