In case projection looks like
(item => new Result { Connection = item.Connection, ConnectionType = item.ConnectionType} )
the method in question returns only one field - Connection
It results in FindProjectionTranslator.VisitField not finding ConnectionType in fields list and it mistakenly sets _fullDocument = true, wich results in using ProjectingDeserializer that deserializes whole (very large) document and slows our system like x10 times.

The fix is proposed here (in April)
https://github.com/mongodb/mongo-csharp-driver/pull/323