diff --git a/lisp/emacs-lisp/regexp-opt.el b/lisp/emacs-lisp/regexp-opt.el
index 6c2350e9548..3edaca78e32 100644
--- a/lisp/emacs-lisp/regexp-opt.el
+++ b/lisp/emacs-lisp/regexp-opt.el
@@ -140,7 +140,7 @@ usually more efficient than that of a simplified version:
 	   (open (cond ((stringp paren) paren) (paren "\\(")))
 	   (re (if strings
                    (regexp-opt-group
-                    (delete-dups (sort (copy-sequence strings) 'string-lessp))
+                    (delete-dups (sort strings))
                     (or open t) (not open))
                  ;; No strings: return an unmatchable regexp.
                  (concat (or open "\\(?:") regexp-unmatchable "\\)"))))
@@ -250,7 +250,7 @@ Merges keywords to avoid backtracking in Emacs's regexp matcher."
 		       (prefixes
 			;; Sorting is necessary in cases such as ("ad" "d").
 			(sort (mapcar (lambda (s) (substring s 0 n)) strings)
-			      'string-lessp)))
+                              :in-place t)))
 		  (concat open-group
 			  (regexp-opt-group prefixes t t)
 			  (regexp-quote (nreverse xiffus))
diff --git a/lisp/emacs-lisp/rx.el b/lisp/emacs-lisp/rx.el
index c512d42cd15..58f95c7d89a 100644
--- a/lisp/emacs-lisp/rx.el
+++ b/lisp/emacs-lisp/rx.el
@@ -581,7 +581,7 @@ a list of named character classes in the order they occur in BODY."
     (cons (rx--condense-intervals
            (sort (append conses
                          (mapcan #'rx--string-to-intervals strings))
-                 #'car-less-than-car))
+                 :key #'car :in-place t))
           (nreverse classes))))
 
 (defun rx--generate-alt (negated intervals classes)