The documentation for write concern j:true (https://docs.mongodb.com/manual/reference/write-concern/#writeconcern.j) is missing details on journal commit interval: https://docs.mongodb.com/manual/reference/configuration-options/#storage.journal.commitIntervalMs.

In particular, j:true results in:

• immediate sync for WT (MongoDB 3.2+)
• commit interval reduced to 1/3 of value for MMAPv1