Investigage speeding up source excludes at index time

[nik9000]

It looks like we talked about speeding up source filtering by going "closer to the metal" in #25168 and never managed to get it to have the same semantics or to really save much time. I think its worth double checking on it - specifically in the context of excludes and includes on the _source field. It feels like we could do better here to me, but we did investigate in #25168 and didn't see it.

If we can't get any speed I figure we should close this and #25168, but I wanted to file this separately from #25168 to track that we're thinking about it at index time too.

[elasticmachine]

Pinging @elastic/es-search (Team:Search)