Oplog getmores can return with 0 entries if there is nothing new in the oplog, rather than waiting for more entries to appear.

Under some circumstances this can increase the latency of w:majority or w:2 operations, because the operation must wait for the round-trip of the getmore that returned 0 as well as the round trip for its oplog entry before getting acknowledgement that the operation has been committed on the secondaries. This impacts for example single-threaded clients that are issuing repeated writes in a loop as fast as possible.