protected function get_progress_trace_instance() {
        if ($this->progress === null) {
            if (!$this->table->is_downloading()) {
                $this->progress = new \core\progress\display_if_slow(get_string('calculatingallstats', 'quiz_statistics'));
                $this->progress->set_display_names();
            } else {
                $this->progress = new \core\progress\none();
            }
        }
        return $this->progress;
    }