Using a list's insert function is much slower than using slice assignment:

> python -m timeit -n 100000 -s "a=[]" "a.insert(0,0)"
100000 loops, best of 5: 19.2 usec per loop

> python -m timeit -n 100000 -s "a=[]" "a[0:0]=[0]"
100000 loops, best of 5: 6.78 usec per loop

(Note that the list starts empty but grows to 100,000 elements.)

At first I thought maybe it's the attribute lookup or function call overhead or so, but inserting near the end shows that that's negligible:

> python -m timeit -n 100000 -s "a=[]" "a.insert(-1,0)"
100000 loops, best of 5: 79.1 nsec per loop

I asked at StackOverflow and someone pointed out that list.insert uses a manual loop instead of memmove: https://stackoverflow.com/a/60466572/12671057