public static function instance() {
        global $CFG;
        if (is_null(self::$singletoninstance)) {
            if (!empty($CFG->perfdebug) and $CFG->perfdebug > 7) {
                self::$singletoninstance = new performance_measuring_filter_manager();
            } else {
                self::$singletoninstance = new self();
            }
        }
        return self::$singletoninstance;
    }