See https://www.elastic.co/guide/en/elasticsearch/reference/current/search-request-source-filtering.html

I think not using source filtering may involve unnecessary high network bandwidth usage when querying indices containing large documents.

We could probably disable the _source attribute altogether most of the time.

If we've got some time, we also may use more fine-grained source filtering when the "SOURCE" projection is not requested, but only a stored field is asked for.

(As explained in HSEARCH-2358 Open , we must use "_source" when retrieving stored field values)