Usually, functional programming involving many computations is best made to take less time using memoization -- the basic idea is that if you feed a true function (something that always evaluates the same result for a given argument) the same thing twice or more, you're wasting time, time that could easily be saved by identifying common calls and storing whatever they evaluate down to into a hash table or other quickly-accessible object. See https://en.wikipedia.org/wiki/Memoization for basic theory. It is well-implemented in Common Lisp.
