Clear[Fib1]
Fib1[0] = 0;
Fib1[1] = 1;
Fib1[n_] := Fib1[n - 1] + Fib1[n - 2]

Timing[Fib1[10]]

Timing[Fib1[30]]

$RecursionLimit = Infinity;
Clear[Fib2]
Fib2[0] = 0;
Fib2[1] = 1;
Fib2[n_] := Fib2[n] = Fib2[n - 1] + Fib2[n - 2]

Timing[Fib2[30]]

?Fib2

Timing[res1 = Fib2[10000];]

Timing[res2 = Fibonacci[10000];]

res2 - res1