diff --git a/core/includes/common.inc b/core/includes/common.inc
index 8f60fe4..3748110 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -4306,6 +4306,12 @@ function drupal_render_cache_by_query($query, $function, $expire = CacheBackendI
  *   @code
  *   DRUPAL_CACHE_PER_PAGE | DRUPAL_CACHE_PER_ROLE
  *   @endcode
+ *   Carefully consider exactly what can cause the content of your block to
+ *   change. If you choose too general a granularity then you could end up
+ *   serving the wrong content, but if you are too specific then you could end
+ *   up with a very large cache and low hit rate, which could actually reduce
+ *   performance. In particular, DRUPAL_CACHE_PER_USER can be very
+ *   resource-consuming on sites with many users.
  *
  * @return
  *   An array of cache ID parts, always containing the active theme. If the
@@ -4325,16 +4331,12 @@ function drupal_render_cid_parts($granularity = NULL) {
   }
 
   if (!empty($granularity)) {
-    // 'PER_ROLE' and 'PER_USER' are mutually exclusive. 'PER_USER' can be a
-    // resource drag for sites with many users, so when a module is being
-    // equivocal, we favor the less expensive 'PER_ROLE' pattern.
     if ($granularity & DRUPAL_CACHE_PER_ROLE) {
       $cid_parts[] = 'r.' . implode(',', $user->getRoles());
     }
-    elseif ($granularity & DRUPAL_CACHE_PER_USER) {
+    if ($granularity & DRUPAL_CACHE_PER_USER) {
       $cid_parts[] = 'u.' . $user->id();
     }
-
     if ($granularity & DRUPAL_CACHE_PER_PAGE) {
       $cid_parts[] = $base_root . request_uri();
     }