That gain is often lost by people doing unoptimized queries all over the place, though, instead of a single place where the queries are optimized. And passing the fat django objects around often lead to accidental n+1 queries, since you can't really trust that looking up a property on your object doesn't do a new query. Often nicer to avoid it all, by having a gated access to the DB.

While I propose most often sending in a list of related IDs (premature optimization and all that), the function could just accept any iterable, and you from the outside could send in the lazy relatedmodel query.