The following issue is against Drupal 8. It is a known performance issue and there is a working patch that is still not in Drupal core:
http://drupal.org/node/1710656

Just before _menu_link_translate($item); in the _menu_tree_check_access() function of /includes/menu.inc:1027 we need:

    // Do not load hidden menu items if not in active breadcrumb trail and
    // user can't administer the menu.
    if (!empty($item['hidden']) && empty($item['in_active_trail']) && !user_access('administer menu')) {
      continue;
    }

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Status: Active » Closed (outdated)

Automatically closed because Drupal 6 is no longer supported. If the issue verifiably applies to later versions, please reopen with details and update the version.