Paste on 2022-02-15T10:23:41

(defun fast-filter (list)
  (mapcar #'cdr
          (remove-duplicates
           (mapcar (lambda (x)
                     (cons (key-fn x) x))
                   list)
           :key #'car
           :from-end t)))

(defun slow-filter (list)
  (remove-duplicates
   list
   :key #'key-fn
   :from-end t))