Currently a ServerSession is acquired for each ClientSession in the constructor, which means that every operation creates a session as soon as its called, potentially much earlier than the operation is actually run. DRIVERS-1030 proposes that server session acquisition be deferred until a connection has been checked out for operation execution. While I anticipate a more comprehensive solution for the Node.js driver when we tackle that ticket, today we can make server session acquisition "lazy" and gain the same benefit.