Ще тупіший алгоритм:

(require 'cl)

(defun fib-loop (num)
  (cl-do ((n 0 (1+ n))
       (cur 0 next)
       (next 1 (+ cur next)))
      ((= num n) cur)))

ELISP> (benchmark 1 (fib-loop 100500))
"Elapsed time: 0.000002s"