I was reading over this page after I was surprised by the size of some of these objects: https://www.boost.org/doc/libs/1_59_0/libs/optional/doc/html/boost_optional/tutorial/performance_considerations.html

It is probably worth doing some of that bool _has_allow_partial_results pattern rather than all the boost::optionals we have here: https://github.com/mongodb/mongo/blob/cf00278a5230148eaf71c04df84f6c6de77decee/src/mongo/db/query/query_stats_find_key_generator.h#L91