diff --git a/includes/registry.inc b/includes/registry.inc
index 29a1fca8cc..d5bc0af794 100644
--- a/includes/registry.inc
+++ b/includes/registry.inc
@@ -19,7 +19,6 @@
  * Does the work for registry_update().
  */
 function _registry_update() {
-
   // The registry serves as a central autoloader for all classes, including
   // the database query builders. However, the registry rebuild process
   // requires write ability to the database, which means having access to the
@@ -33,6 +32,11 @@ function _registry_update() {
   require_once DRUPAL_ROOT . '/includes/database/select.inc';
   require_once DRUPAL_ROOT . '/includes/database/' . $driver . '/query.inc';
 
+  // During the first registry rebuild in a request, we check all the files.
+  // During subsequent rebuilds, we only add new files. It makes the rebuilding
+  // process faster during installation of modules.
+  static $check_existing_files = TRUE;
+
   // Get current list of modules and their files.
   $modules = db_query("SELECT * FROM {system} WHERE type = 'module'")->fetchAll();
   // Get the list of files we are going to parse.
@@ -55,6 +59,9 @@ function _registry_update() {
     $files["$filename"] = array('module' => '', 'weight' => 0);
   }
 
+  // Initialize an empty array for the unchanged files.
+  $unchanged_files = array();
+
   $transaction = db_transaction();
   try {
     // Allow modules to manually modify the list of files before the registry
@@ -63,10 +70,19 @@ function _registry_update() {
     // list can then be added to the list of files that the registry will parse,
     // or modify attributes of a file.
     drupal_alter('registry_files', $files, $modules);
+
     foreach (registry_get_parsed_files() as $filename => $file) {
       // Add the hash for those files we have already parsed.
       if (isset($files[$filename])) {
-        $files[$filename]['hash'] = $file['hash'];
+        if ($check_existing_files === TRUE) {
+          $files[$filename]['hash'] = $file['hash'];
+        }
+        else {
+          // Ignore that file for this request, it has been parsed previously
+          // and it is unlikely it has changed.
+          unset($files[$filename]);
+          $unchanged_files[$filename] = $file;
+        }
       }
       else {
         // Flush the registry of resources in files that are no longer on disc
@@ -79,8 +95,12 @@ function _registry_update() {
           ->execute();
       }
     }
+
     $parsed_files = _registry_parse_files($files);
 
+    // Add unchanged files to the files.
+    $files += $unchanged_files;
+
     $unchanged_resources = array();
     $lookup_cache = array();
     if ($cache = cache_get('lookup_cache', 'cache_bootstrap')) {
@@ -89,7 +109,7 @@ function _registry_update() {
     foreach ($lookup_cache as $key => $file) {
       // If the file for this cached resource is carried over unchanged from
       // the last registry build, then we can safely re-cache it.
-      if ($file && in_array($file, array_keys($files)) && !in_array($file, $parsed_files)) {
+      if ($file && isset($files[$file]) && !in_array($file, $parsed_files, TRUE)) {
         $unchanged_resources[$key] = $file;
       }
     }
@@ -102,6 +122,10 @@ function _registry_update() {
     throw $e;
   }
 
+  // During the next run in this request, don't bother re-checking existing
+  // files.
+  $check_existing_files = FALSE;
+
   // We have some unchanged resources, warm up the cache - no need to pay
   // for looking them up again.
   if (count($unchanged_resources) > 0) {