SICP - exercise 1.12 - pascal's triangle
You should memoise the recursive call in order to make it faster. Since each value has a computational sibling (its value is derived from (one or two) parents which also each has another child), you remove the double-computation that would occur.

Additionally, these double-computations would actually accumulate exponentially up the levels, so it's no longer just a micro-optimisation; it actually affects the your runtime complexity. So it's even more important to implement memoisation.