Because of memoization, each Fibonacci number is only computed once. The first
two Fibonacci numbers require 0 additions to compute because they are
hard-coded. Each following Fibonacci number requires an additional addition.
Thus computing up to the nth Fibonacci number requires n - 2 additions. Repeated
accesses to the same number in the sequence require 0 further additions.

Without memorization accessing the nth item in the sequence requires >2 times as
many additions as accessing the n - 2 th. Thus number of additions grows
exponentially.