/**
 * Implements hook_views_pre_execute().
 */
function {{ machine_name }}_views_pre_execute(ViewExecutable $view) {
  // Whenever a view queries more than two tables, show a message that notifies
  // view administrators that the query might be heavy.
  // (This action could be performed later in the execution process, but not
  // earlier.)
  $account = \Drupal::currentUser();

  if (count($view->query->tables) > 2 && $account->hasPermission('administer views')) {
    \Drupal::messenger()->addWarning(t('The view %view may be heavy to execute.', ['%view' => $view->id()]));
  }
}