Flatten list in a loop

(defun flatten (x &optional y)
  (declare (optimize (speed 3) (safety 0)))
  (cond
    ((null x)
     (cond
       ((null y) nil)
       ((listp (car y))
        (flatten (car y) (cdr y)))
       (t (cons (car y) (flatten (cdr y))))))
    ((listp (car x))
     (flatten (car x) (if y (list (cdr x) y) (cdr x))))
    (t (cons (car x) (flatten (cdr x) y)))))

(defun flatten-1 (l)
  (declare (optimize (speed 3) (safety 0)))
  (cond
    ((null l) nil)
    ((atom l) (list l))
    (t (loop for a in l appending (flatten-1 a)))))

(defun flatten-2 (l)
  (declare (optimize (speed 3) (safety 0)))
  (when l
    (if (atom l)
    (list l)
    (mapcan #'flatten-2 l))))

(defun flatten-3 (x)
  (declare (optimize (speed 3) (safety 0)))
  (do ((a x))
      ((null a))
    (if (listp (car a))
    (do ((b (car a) (cdr b))
         (c a)
         (d (cdr a)))
        ((null b)
         (and (setf (cdr c) d) nil))
      (setf (car c) (car b)
        (cdr c) (cdr b)
        c (or (cdr c) c)))
    (setf a (cdr a)))) x)

;; 353
;; 2056
;; 1230
;; 63

(let ((test (get-internal-real-time)))
  (dotimes (i 1e6)
    (flatten '(1 2 (3 4) (5 (6 (7 8) 9 a (10))) b (10.5 (1.6) (1.7)) 11)))
  (print (- (get-internal-real-time) test))
  (setf test (get-internal-real-time))
  (dotimes (i 1e6)
    (flatten-2 '(1 2 (3 4) (5 (6 (7 8) 9 a (10))) b (10.5 (1.6) (1.7)) 11)))
  (print (- (get-internal-real-time) test))
  (setf test (get-internal-real-time))
  (dotimes (i 1e6)
    (flatten-1 '(1 2 (3 4) (5 (6 (7 8) 9 a (10))) b (10.5 (1.6) (1.7)) 11)))
  (print (- (get-internal-real-time) test))
  (setf test (get-internal-real-time))
  (dotimes (i 1e6)
    (flatten-3 '(1 2 (3 4) (5 (6 (7 8) 9 a (10))) b (10.5 (1.6) (1.7)) 11)))
  (print (- (get-internal-real-time) test)))