Every stock IPO spikes, of course this will lead to atleast a temporary price spike.
def product_sum(a, b):
total = 0
for a, b in zip(a, b):
total += a * b
return total
%timeit product_sum(million_a, million_b)
215 ms ± 3.01 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
%timeit (million_a * million_b).sum()
6.01 ms ± 254 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)