Maybe I misunderstands, but .setFetchSource is using source filtering right?
In that case, that is what we are doing know, and gives us problems with slow queries, as the documents has to be parsed, to find the fields which should be returned.
I have tried to make the fields stored, but this seems like a bad solution, when we have to make many fields stored, and make tricks to save object relations - since these are lost in stored fields.
In addition to this, it is slow to extract from mulitple stored fields, compared to a source field in a smaller index.
I have not tested doc_values - maybe this could be a solution.
In this discussion Martijn Van Groningen explains that stored fields should be used for returning field values, while doc_values are for fast retrieving doing scripts and queries. which is also how I understood it from the documentations.
Is there a reason you suggest using doc_values as a solution, before using stored fields?
And why is it a problem to use doc_values if we want to retrieve multiple fields?