Visualization - Fibonacci Tree

Before you look at this tree, make a prediction: how many times do you think $fib(2)$ gets calculated when computing $fib(5)$? Once? Twice? More? Hold that guess.

Here's the call tree for $fib(5)$: Notice $fib(3)$ computed twice, $fib(2)$ three times, $fib(1)$ five times! You're doing the same work over and over. This is the main problem you need to fix. Tracing through examples helps build intuition for how it works.