Index: includes/module.inc
===================================================================
--- includes/module.inc        (revision 0)
+++ includes/module.inc        (working copy)
@@ -403,8 +403,29 @@
  */
 function module_implements($hook, $sort = FALSE, $refresh = FALSE) {
   static $implementations;
+  static $cache_exists;
 
+  if (!isset($cache_exists)) {
+    $cache_exists = function_exists('cache_get');
+  }
+
+  // Load implementations from cache.
+  if (!isset($implementations)) {
+    if ($cache_exists) {
+      $cache = cache_get('module_implements');
+      if ($cache) {
+        $implementations = $cache->data;
+      }
+      else {
+        $implementations = array();
+      }
+    }
+  }
+
   if ($refresh) {
+    if ($cache_exists) {
+      cache_clear_all('module_implements', 'cache');
+    }
     $implementations = array();
     return;
   }
@@ -417,6 +438,10 @@
         $implementations[$hook][] = $module;
       }
     }
+
+    if ($cache_exists) {
+      cache_set('module_implements', $implementations);
+    }
   }
 
   // The explicit cast forces a copy to be made. This is needed because