I tried it with a couple of different thresholds, twice each, ignoring the results of the first run. 16 seems to be the sweet spot.

THRESHOLD = 0
jeethu@dev:cpython  (3.7_list_insert_memmove)$ ./python -m timeit -s "l = []" "for _ in range(100): l.insert(0, None)"
500 loops, best of 5: 787 usec per loop

THRESHOLD = 4
jeethu@dev:cpython  (3.7_list_insert_memmove)$ ./python -m timeit -s "l = []" "for _ in range(100): l.insert(0, None)"
500 loops, best of 5: 781 usec per loop

THRESHOLD = 8
jeethu@dev:cpython  (3.7_list_insert_memmove)$ ./python -m timeit -s "l = []" "for _ in range(100): l.insert(0, None)"
500 loops, best of 5: 780 usec per loop

THRESHOLD = 16
jeethu@dev:cpython  (3.7_list_insert_memmove)$ ./python -m timeit -s "l = []" "for _ in range(100): l.insert(0, None)"
500 loops, best of 5: 758 usec per loop

THRESHOLD = 32
jeethu@dev:cpython  (3.7_list_insert_memmove)$ ./python -m timeit -s "l = []" "for _ in range(100): l.insert(0, None)"
500 loops, best of 5: 764 usec per loop