Only size matters?
I have tested tic toc time for 4 kinds of solutions I have seen for this problem. I have computed 1e5 fibonacci numbers f(n) with n<=70 picked at random. Here are the results:
1) Explicit formula (sltn 408159, size 42): 3.83e-01 s. \\
2) for loop (sltn 409425, size 36): 1.09e-01 s. \\
3) filter (sltn 409380, size 33): 4.78e-01 s. \\
4) Recursion (sltn 408916, size 31): inifinity