Call to variable_del('menu_rebuild_needed') sometimes takes forever

I'm seeing this occasionally in New Relic:

And upon inspection find this:

The culprit seems to be the variable_delete('menu_rebuild_needed') call in subpathauto_url_inbout_alter():

/**
 * Implements hook_url_inbound_alter().
 */
function subpathauto_url_inbound_alter(&$path, $original_path, $language) {
  static $drupal_static_fast;

  if (!isset($drupal_static_fast)) {
    $drupal_static_fast['items'] = &drupal_static(__FUNCTION__, array());
  }

  $items = &$drupal_static_fast['items'];
  // If the current menu item exists at this path, then we should not continue
  // processing.
  if (empty($items[$path])) {
    $menu_rebuild_needed = variable_get('menu_rebuild_needed', FALSE);
    // In no cases rebuild the menu while trying to load the menu item here
    // since that would cause infinite loops if any module calls
    // drupal_get_normal_path() in hook_link_alter().
    variable_del('menu_rebuild_needed');
    $items[$path] = menu_get_item($path);
    if ($menu_rebuild_needed) {
      variable_set('menu_rebuild_needed', TRUE);
    }
  }
  if (!empty($items[$path]) && $items[$path]['href'] == $path) {
    return FALSE;
  }

  if ($source = subpathauto_lookup_subpath('source', $path, $original_path, $language)) {
    $path = $source;
  }
}

The first time I saw this I thought it was a random outlier, but I've now seen it multiple times, happening on normal page views.

Any thoughts?