diff -u b/core/modules/toolbar/js/toolbar.js b/core/modules/toolbar/js/toolbar.js
--- b/core/modules/toolbar/js/toolbar.js
+++ b/core/modules/toolbar/js/toolbar.js
@@ -49,6 +49,9 @@
     $toolbar = $(context).find('#toolbar-administration').once('toolbar');
     if ($toolbar.length) {
       // Add subtrees.
+      // @todo Optimize this to delay adding each subtree to the DOM until it is
+      //   needed; however, take into account screen readers for determining
+      //   when the DOM elements are needed.
       if (Drupal.toolbar.subtrees) {
         for (var id in Drupal.toolbar.subtrees) {
           $('#toolbar-link-' + id).after(Drupal.toolbar.subtrees[id]);
diff -u b/core/modules/toolbar/toolbar.module b/core/modules/toolbar/toolbar.module
--- b/core/modules/toolbar/toolbar.module
+++ b/core/modules/toolbar/toolbar.module
@@ -240,7 +240,11 @@
     '#heading' => t('Administration menu'),
   );
 
-  // @todo Add a comment explaining this.
+  // To conserve bandwidth, we only include the top-level links in the HTML.
+  // The subtrees are included in a JSONP script, cached by the browser. Here we
+  // add that JSONP script. We add it as an external script, because it's a
+  // Drupal path, not a file available via a stream wrapper.
+  // @see toolbar_subtrees_jsonp()
   $menu['toolbar_administration']['#attached']['js'][url('toolbar/subtrees/' . _toolbar_get_subtree_hash())] = array('type' => 'external');
 
   $items['administration'] = array(