A really nasty solution to prevent call of TYPO3 Solr getAvailableSites performance issue

CopyPageTreeHook.php

    /**
     * While copying pagetree we have to prevent calling getAvailableSites of solr, as it
     * initializes ALL 300 pagetrees and load its full TS.
     */
    protected function modifyRequestForSolrAndSolrfal()
    {
        // While copying pagetree solr will monitor various records. But without known
        // rootPageUid it will call getAllSites() which will build the TS for all sites.
        // We set monitored tables to invalidTable, so NO table will be monitored.
        $solrExtConf = unserialize($GLOBALS['TYPO3_CONF_VARS']['EXT']['extConf']['solr']);
        $solrExtConf['useConfigurationMonitorTables'] = 'invalidTable';
        $GLOBALS['TYPO3_CONF_VARS']['EXT']['extConf']['solr'] = serialize($solrExtConf);

        // While copying pagetree solrfal will check, if a record is used on another pagetree.
        // To prevent that, we have to add all known tables temporary to siteExclusiveRecordTables
        $connectionPool = GeneralUtility::makeInstance(ConnectionPool::class);
        $connection = $connectionPool->getConnectionByName(ConnectionPool::DEFAULT_CONNECTION_NAME);
        $tables = $connection->getSchemaManager()->listTableNames();

        $solrfalExtConf = unserialize($GLOBALS['TYPO3_CONF_VARS']['EXT']['extConf']['solrfal']);
        $solrfalExtConf['siteExclusiveRecordTables'] = implode(',', $tables);
        $GLOBALS['TYPO3_CONF_VARS']['EXT']['extConf']['solrfal'] = serialize($solrfalExtConf);
    }