In [2]: %time fib(40)
CPU times: user 2.24 s, sys: 27.3 ms, total: 2.27 s
Wall time: 2.27 s
Out[2]: 165580141

In [3]: %time fib_tail(40)
CPU times: user 254 µs, sys: 61 µs, total: 315 µs
Wall time: 330 µs
Out[3]: 165580141