Thanks for confirming @m4kkur0. If memory usage is your main concern I would recommend using docvalue_fields. There is a bit more detail in this section of the documentation that discusses the loading of the document when using the _source attribute.
With _source usage the entire Lucene document will be loaded:
A document’s
_sourceis stored as a single field in Lucene. This structure means that the whole_sourceobject must be loaded and parsed even if you’re only requesting part of it.