No. The default write concern as of version 5 is "majority" which "Requests acknowledgment that write operations have been durably committed to the calculated m

No. The default write concern as of version 5 is "majority" which "Requests acknowledgment that write operations have been durably committed to the calculated majority of the data-bearing voting members" (as long as writeConcernMajorityJournalDefault is true which is the default). So fsync is called on every write.