wow, TIL!

    Python 3.13.5 (main, Jul 15 2026, 20:25:40) [GCC 14.2.0] on linux
    >>> x = [[n]*1000 for n in range(1000)]; import timeit, itertools, functools, operator
    >>> timeit.timeit("len(list(sum(x, [])))", number=10, globals=globals())
    13.009033881127834
    >>> timeit.timeit("len(list(list(functools.reduce(operator.add, x, []))))", number=10, globals=globals())
    12.941937348805368
    >>> timeit.timeit("len(list(itertools.chain.from_iterable(x)))", number=10, globals=globals())
    0.0706032607704401
    >>> timeit.timeit("out=[]; [out.extend(i) for i in x]; len(out)", number=10, globals=globals())
    0.06334403157234192
    >>> timeit.timeit("len([i for a in x for i in a])", number=10, globals=globals())
    0.1232151910662651

mutable is fastest, itertools is just a bit slower, list comprehension is 2x slower, both "sum(..., [])" and "reduce" are 200 times slower!