Currently, \eZ\Publish\Core\Search\Common\Slot\AbstractSubtree::indexSubtree iterates over the list of locations and calls \eZ\Publish\SPI\Search\Handler::indexContent which is not optimal performance wise.
Therefore, \eZ\Publish\Core\Search\Common\Slot\AbstractSubtree::indexSubtree should be changed and bulkIndexContent should be used instead (currently implemented in \EzSystems\EzPlatformSolrSearchEngine\Handler only). A new interface, like BulkIndexInterface should be introduced for master while for existing LTSes it could be handled via method_exists check for instance.