Index: includes/theme.inc
===================================================================
--- includes/theme.inc	(revision 4505)
+++ includes/theme.inc	(working copy)
@@ -175,8 +175,9 @@
       include_once './'. $theme->owner;
     }
   }
-
-  $registry_callback($theme, $base_theme, $theme_engine);
+  // Do not load the full theme registry by default.
+  // @see theme_get_hook().
+  //$registry_callback($theme, $base_theme, $theme_engine);
 }
 
 /**
@@ -598,33 +599,34 @@
  *   An HTML string that generates the themed output.
  */
 function theme() {
+  static $initialized;
   $args = func_get_args();
   $hook = array_shift($args);
 
   static $hooks = NULL;
-  if (!isset($hooks)) {
+  if (!isset($initialized)) {
+    $initialized = TRUE;
     init_theme();
-    $hooks = theme_get_registry();
   }
 
   if (is_array($hook)) {
     foreach ($hook as $candidate) {
-      if (isset($hooks[$candidate])) {
+      if (theme_get_hook($candidate)) {
         break;
       }
     }
     $hook = $candidate;
   }
 
-  if (!isset($hooks[$hook])) {
+  if (!theme_get_hook($hook)) {
     return;
   }
 
-  $info = $hooks[$hook];
+  $info = theme_get_hook($hook);
   global $theme_path;
   $temp = $theme_path;
   // point path_to_theme() to the currently used theme path:
-  $theme_path = $hooks[$hook]['theme path'];
+  $theme_path = $info['theme path'];
 
   // Include a file if the theme function or preprocess function is held elsewhere.
   if (!empty($info['include files'])) {
@@ -677,7 +679,7 @@
     if (isset($theme_engine)) {
       // If theme or theme engine is implementing this, it may have
       // a different extension and a different renderer.
-      if ($hooks[$hook]['type'] != 'module') {
+      if ($info['type'] != 'module') {
         if (function_exists($theme_engine .'_render_template')) {
           $render_function = $theme_engine .'_render_template';
         }
@@ -717,9 +719,9 @@
     }
 
     if (empty($template_file)) {
-      $template_file = $hooks[$hook]['template'] . $extension;
-      if (isset($hooks[$hook]['path'])) {
-        $template_file = $hooks[$hook]['path'] .'/'. $template_file;
+      $template_file = $info['template'] . $extension;
+      if ($info['path']) {
+        $template_file = $info['path'] .'/'. $template_file;
       }
     }
     $output = $render_function($template_file, $variables);
@@ -734,6 +736,44 @@
 }
 
 /**
+ * Find the theme definition for a hook.
+ *
+ * This function maintains a cached subset of the theme registry which is built
+ * during calls to theme. To avoid loading the entire theme registry on every
+ * request for hooks which may not be called during normal site usage.
+ *
+ * @param $hook
+ *   The theme hook being requested.
+ * @return
+ *   The registry entry for the hook, or FALSE if none was found.
+ */
+function theme_get_hook($hook) {
+  global $theme;
+  static $hooks = array();
+  if (empty($hooks)) {
+    if ($cached = cache_get("theme_registry:hooks:$theme")) {
+      $hooks = $cached->data;
+    }
+  }
+  if (isset($hooks[$hook])) {
+    return $hooks[$hook];
+  }
+  else {
+    $registry = theme_get_registry();
+    if (isset($registry[$hook])) {
+      $hooks[$hook] = $registry[$hook];
+      cache_set("theme_registry:hooks:$theme", $hooks);
+      return $hooks[$hook];
+    }
+    else {
+      $hooks[$hook] = FALSE;
+      cache_set("theme_registry:hooks:$theme", $hooks);
+    }
+  }
+  return FALSE;
+}
+
+/**
  * Choose which template file to actually render. These are all suggested
  * templates from themes and modules. Theming implementations can occur on
  * multiple levels. All paths are checked to account for this.