diff --git a/include/stdlib.h b/include/stdlib.h
--- a/include/stdlib.h
+++ b/include/stdlib.h
@@ -108,7 +108,7 @@
 size_t	 mbstowcs(wchar_t * __restrict , const char * __restrict, size_t);
 int	 mbtowc(wchar_t * __restrict, const char * __restrict, size_t);
 void	 qsort(void *, size_t, size_t,
-	    int (* _Nonnull)(const void *, const void *));
+	    int (* _Nonnull)(const void *, const void *)) __may_be_slow;
 int	 rand(void);
 void	*realloc(void *, size_t) __result_use_check __alloc_size(2);
 void	 srand(unsigned);
@@ -301,7 +301,7 @@
 int	 heapsort_b(void *, size_t, size_t,
 	    int (^ _Nonnull)(const void *, const void *));
 void	 qsort_b(void *, size_t, size_t,
-	    int (^ _Nonnull)(const void *, const void *));
+	    int (^ _Nonnull)(const void *, const void *)) __may_be_slow;
 #endif
 int	 l64a_r(long, char *, int);
 int	 mergesort(void *, size_t, size_t, int (*)(const void *, const void *));
@@ -312,7 +312,7 @@
 int	 mkostemps(char *, int, int);
 int	 mkostempsat(int, char *, int, int);
 void	 qsort_r(void *, size_t, size_t,
-	    int (*)(const void *, const void *, void *), void *);
+	    int (*)(const void *, const void *, void *), void *) __may_be_slow;
 int	 radixsort(const unsigned char **, int, const unsigned char *,
 	    unsigned);
 void	*reallocarray(void *, size_t, size_t) __result_use_check
@@ -349,7 +349,7 @@
  */
 #if defined(__generic) || defined(__cplusplus)
 void __qsort_r_compat(void *, size_t, size_t, void *,
-	    int (*)(void *, const void *, const void *));
+	    int (*)(void *, const void *, const void *)) __may_be_slow;
 __sym_compat(qsort_r, __qsort_r_compat, FBSD_1.0);
 #endif
 #if defined(__generic) && !defined(__cplusplus)
@@ -360,7 +360,7 @@
 __END_DECLS
 extern "C++" {
 static inline void qsort_r(void *base, size_t nmemb, size_t size,
-    void *thunk, int (*compar)(void *, const void *, const void *)) {
+    void *thunk, int (*compar)(void *, const void *, const void *)) __may_be_slow {
 	__qsort_r_compat(base, nmemb, size, thunk, compar);
 }
 }
@@ -394,7 +394,7 @@
 void ignore_handler_s(const char * __restrict, void * __restrict, errno_t);
 /* K.3.6.3.2 */
 errno_t	 qsort_s(void *, rsize_t, rsize_t,
-    int (*)(const void *, const void *, void *), void *);
+    int (*)(const void *, const void *, void *), void *) __may_be_slow;
 #endif /* __EXT1_VISIBLE */
 
 __END_DECLS
diff --git a/sys/sys/cdefs.h b/sys/sys/cdefs.h
--- a/sys/sys/cdefs.h
+++ b/sys/sys/cdefs.h
@@ -879,6 +879,15 @@
 /* Function should not be analyzed. */
 #define	__no_lock_analysis	__lock_annotate(no_thread_safety_analysis)
 
+/*
+ * The function marked by this macro may execute slower than expected.
+ */
+#if __has_attribute(warning)
+#define	__may_be_slow		__attribute__((warning("This function may execute slower than expected.")))
+#else
+#define	__may_be_slow
+#endif
+
 /*
  * Function or variable should not be sanitized, e.g., by AddressSanitizer.
  * GCC has the nosanitize attribute, but as a function attribute only, and