When decoding commands, the 3.x driver double decodes the reply document. The first time it decodes into a BsonDocument just to check for command failure. The second time it decodes into an instance of type T, which is unknown at compile time and can't be used to check for failure.

This has become a significant performance regression when connected to a 3.2 server, since as of 3.2 the driver uses the find and getMore commands for all queries and all get-more messages.