The ChangeStreamAddPostImageStage looks up fields in the input document redundantly, e.g.

• fullDocumentBeforeChange up to 3 times.
• rawOplogUpdateSpec up to 2 times

The Document class adds a field value to a cache once the field has been looked up once, so repeated lookups should normally be fast. However, the Document class uses a linear scan over the cached fields if the number of looked up fields is <= 4, so there can still be some penalty. It is best to avoid repeated lookups of the same field altogether.

Additionally, the class converts string field values to std::string s unnecessarily in a few cases (e.g. here and here). This should be converted to use StringData instead.