Core

static public function getPagesTSconfig($id, $rootLine = '', $returnPartArray = 0) {
        $id = intval($id);
        if (!is_array($rootLine)) {
            $rootLine = self::BEgetRootLine($id, '', TRUE);
        }
        // Order correctly
        ksort($rootLine);
        $TSdataArray = array();
        // Setting default configuration
        $TSdataArray['defaultPageTSconfig'] = $GLOBALS['TYPO3_CONF_VARS']['BE']['defaultPageTSconfig'];
        foreach ($rootLine as $k => $v) {
            $TSdataArray['uid_' . $v['uid']] = $v['TSconfig'];
        }
        $TSdataArray = TypoScriptParser::checkIncludeLines_array($TSdataArray);
        if ($returnPartArray) {
            return $TSdataArray;
        }
        // Parsing the page TS-Config
        $pageTS = implode(LF . '[GLOBAL]' . LF, $TSdataArray);
        /* @var $parseObj \TYPO3\CMS\Backend\Configuration\TsConfigParser */
        $parseObj = GeneralUtility::makeInstance('TYPO3\\CMS\\Backend\\Configuration\\TsConfigParser');
        $res = $parseObj->parseTSconfig($pageTS, 'PAGES', $id, $rootLine);
        if ($res) {
            $TSconfig = $res['TSconfig'];
        }
        // Get User TSconfig overlay
        $userTSconfig = $GLOBALS['BE_USER']->userTS['page.'];
        if (is_array($userTSconfig)) {
            $TSconfig = GeneralUtility::array_merge_recursive_overrule($TSconfig, $userTSconfig);
        }
        return $TSconfig;
    }

While parseTSconfig already should make use of a cache, which is not working properly at the moment, we could still improve this method by creating TSconfig only once per request. This does not have to make use of the fully fledged caching framework but could use a variable to return it early.

This way we could get rid of having to do array_merge_recursive_overrule umpteen times during a single request.