FWIW, the number of recursive calls to compute fib(n) is fib(n+1). So, expect fib(50) to take approxímately 20365011074/165580141 (roughly 123) times longer tha

FWIW, the number of recursive calls to compute fib(n) is fib(n+1). So, expect fib(50) to take approxímately 20365011074/165580141 (roughly 123) times longer than fib(40) took (this is taking the somewhat optimistic assumption that there's no actual slow-down from the larger bignums, this is NOT a safe assumption).