Currently the Journaling and the WiredTiger Storage Engine page does not mention the effect on WiredTiger if MongoDB is started with the nojournal option.

One of the effect is every journal sync will become a checkpoint instead to satisfy durability requirement.

Since the journal was designed with quick sync in mind, the forced checkpoints could lead to slower performance when durability is required such as in a replica set.

Reference (MongoDB 3.4.9): https://github.com/mongodb/mongo/blob/r3.4.9/src/mongo/db/storage/wiredtiger/wiredtiger_session_cache.cpp#L242-L249