This is coming quite late, but I wanted to link to a new reference page we're adding that gives some guidance on when to use source filtering vs.
How should I do: just turn off the _source and take the values using docvalue_field, or storing _source and get values from it using source filtering?
If you really only need one value in the response, and it's available from doc values, then it could be more efficient to load it using
docvalue_fields instead of source filtering. If you don't use the _source elsewhere, then you could disable it to save space. It's important to know that certain features will no longer work when the _source isn't stored, such as reindex, source-only snapshots, and highlighting (in some cases).