Indeed! I incorrectly thought I tested that under both versions. And I didn't notice the O() performance, which is:

    >>> for i in range(20):
    ...   t1 = time.time()
    ...   n = len(sum((list(range(i)) for i in range(2**i)), []))
    ...   t2 = time.time()
    ...   print(i, t2-t1)
    ... 
    0 0.00010800361633300781
    1 2.288818359375e-05
    2 2.5033950805664062e-05
    3 4.00543212890625e-05
    4 7.295608520507812e-05
    5 0.00017499923706054688
    6 0.0005929470062255859
    7 0.0024929046630859375
    8 0.012469053268432617
    9 0.14577507972717285
    10 1.7278220653533936
    11 17.82376503944397
    ^C

(MHO too, BTW.)