MatchProcessor::process() is called with a Document input for each document that the match expression is evaluated on.
MatchProcessor::process() immediately creates a DocumentMatchableDocument here in order to evaluate the match expression.
DocumentMatchableDocument converts the underlying Document to a BSONObj in its constructor here, so in order to evaluate a match expression there is an unconditional Document to BSONObj conversion.
This conversion should be avoided because it negatively affects match expression performance.