A significant amount of the time in load.php is spent in MediaWiki WebStart.php, via wmf-config/CommonSettings. The biggest one within that seems to be Wikibase providing some values for an Echo hook. About 30x longer compared to Thanks, Flow and EducationProgram for the same Echo hook.

/includes/WebStart.php > EchoHooks::initEchoExtension > Wikibase\Client\Hooks\EchoNotificationsHandlers::onBeforeCreateEchoEvent (git-src)

public static function onBeforeCreateEchoEvent( .. ) {
	$self = self::newFromGlobalState();
	$self->doBeforeCreateEchoEvent( $notifications, $notificationCategories, $icons );
}

The actual call to doBeforeCreateEchoEvent() is runs too quickly to show up. It's mostly setting up the global instance. It seems some of the constructors involved do a lot of (presumably unnecessary) work in the constructor.

• Wikibase\Client\WikibaseClient::getDefaultInstance
  • MediaWiki\MediaWikiServices::getSiteLookup
    • MediaWiki\MediaWikiServices::getSiteStore
      • MediaWiki\MediaWikiServices::getDBLoadBalancer
        • Wikimedia\Rdbms\LBFactoryMulti::getMainLB
          • Wikimedia\Rdbms\ChronologyProtector::initLB
            • Wikimedia\Rdbms\ChronologyProtector::initPositions