由于您仅测量startswith返回True的情况，因此该比较是不公平的。

>>> x = 'foobar'

>>> y = 'fool'

>>> %timeit x.startswith(y)

1000000 loops, best of 3: 221 ns per loop

>>> %timeit x[:3] == y # note: length mismatch

10000000 loops, best of 3: 122 ns per loop

>>> %timeit x[:4] == y

10000000 loops, best of 3: 158 ns per loop

>>> %timeit x[:len(y)] == y

1000000 loops, best of 3: 210 ns per loop

>>> sw = x.startswith

>>> %timeit sw(y)

10000000 loops, best of 3: 176 ns per loop

另外，对于更长的字符串，startswith快得多：

>>> import random

>>> import string

>>> x = '%030x' % random.randrange(256**10000)

>>> len(x)

20000

>>> y = r[:4000]

>>> %timeit x.startswith(y)

1000000 loops, best of 3: 211 ns per loop

>>> %timeit x[:len(y)] == y

1000000 loops, best of 3: 469 ns per loop

>>> sw = x.startswith

>>> %timeit sw(y)

10000000 loops, best of 3: 168 ns per loop

没有比赛时仍然如此。

# change last character of y

>>> y = y[:-1] + chr((ord(y[-1]) + 1) % 256)

>>> %timeit x.startswith(y)

1000000 loops, best of 3: 210 ns per loop

>>> %timeit x[:len(y)] == y

1000000 loops, best of 3: 470 ns per loop

>>> %timeit sw(y)

10000000 loops, best of 3: 168 ns per loop

# change first character of y

>>> y = chr((ord(y[0]) + 1) % 256) + y[1:]

>>> %timeit x.startswith(y)

1000000 loops, best of 3: 210 ns per loop

>>> %timeit x[:len(y)] == y

1000000 loops, best of 3: 442 ns per loop

>>> %timeit sw(y)

10000000 loops, best of 3: 168 ns per loop

因此，对于短字符串，startswith可能较慢，因为它针对长字符串进行了优化。

(技巧是从此答案中获取随机字符串。)