Description of Drivers Ticket:

Clients will be able to specify a new argument to isMaster which speculatively includes the first saslStart command of an authentication handshake. This command may be provided to the server in parallel with any standard request for supported authentication mechanisms. This would permit clients to merge their saslStart message with their isMaster request, and receive the saslStart reply with the isMaster reply. Speculative authentication is ignored for server versions less than or equal to 4.2 and is supported for X.509, SCRAM-SHA-1 and SCRAM-SHA-256.
See DRIVERS-918 for updated details.