OP_MSG supports using payload type 1 (aka document sequence) for the "documents" argument to the insert command. Using this ovoids the string-based overhead of BSON arrays that is incurred when using a regular insert command. We should investigate whether using a document sequence instead would have meaningful performance benefits.

source: https://github.com/mongodb/specifications/blob/master/source/message/OP_MSG.rst#command-arguments-as-payload