When you delete an event, the asset manager method deleteOrphanedProperties() is called, which runs the query:
DELETE p FROM oc_assets_properties p LEFT JOIN oc_assets_snapshot e ON p.mediapackage_id = e.mediapackage_id WHERE e.id IS NULL;
This is slow because it does almost a full table scan on oc_assets_properties, which can have millions of rows.
This doesn't seem necessary, as the properties should get deleted for a specific mediapackage when that MP is deleted.
At best, it's a maintenance task that could get run once a day.