(defun test2 (s1 s2)
  (declare (optimize speed))
  (cond ((typep s1 'a)
         (cond ((typep s2 'a) ...)
               ((typep s2 'b) ...)
               (...)))
        ((typep s1 'b)
         (cond ((typep s2 'a) ...)
               ((typep s2 'b) ...)
               (...)))
        (...)))