Merge duplicates is excruciatingly slow on contacts with multiple custom field entries due to cross join

When rendering the dedupe page the join in

CRM_Core_BAO_CustomGroup::getTree

joins all relevant custom tables on all others - meaning that an contact with 30 multiple custom value fields across 6 tables returns over 3000 rows (& php parses them all)

Query joins look like

LEFT JOIN civicrm_value_campaign_fields_65 USING (entity_id)
LEFT JOIN civicrm_value_major_donor_information_67 USING (entity_id)
LEFT JOIN civicrm_value_event_quiz_night_68 USING (entity_id)
LEFT JOIN civicrm_value_demographic_extra_16 USING (entity_id)
WHERE civicrm_value_polygons_69.entity_id = 33763

Code that generates looks like

if ($firstTable) {
$fromSQL = $firstTable;
foreach ($from as $table) {
if ($table != $firstTable)

{ $fromSQL .= "\nLEFT JOIN $table USING (entity_id)"; }

}

$query = "
SELECT $select
FROM $fromSQL
WHERE {$firstTable}.entity_id = $entityID
";

$dao = CRM_Core_DAO::executeQuery($query);

This is called from lots of places but we only observed slowness here - not sure why. Has anyone hit it?