(defun factorial (n)
  (labels ((helper (n i acc) 
              (if (< n i)
                  acc 
                  (helper n (1+ i) (* i acc)))))
    (helper n 1 1)))



CL-USER> (time (progn (factorial 100000) nil))

Evaluation took:
  1.711 seconds of real time
  1.719623 seconds of total run time (1.715548 user, 0.004075 system)
  [ Run times consist of 0.069 seconds GC time, and 1.651 seconds non-GC time. ]
  100.53% CPU
  5,985,246,295 processor cycles
  9,030,022,672 bytes consed