We recently migrated one of our replicasets to 3.6.3 from 3.4. It had been running just fine for about 4 days when we had a rash of slow queries out of nowhere, the database became super-slow to unresponsive. During that time, the only indication of problems other than log data showing slow queries were the following messages (by the ton):

Line 181225: 2018-04-16T07:28:10.035-0500 E STORAGE [conn908492] WiredTiger error (22) [1523881690:35506][8828:0x7fbc77746700], WT_SESSION.begin_transaction: read timestamp 5ad496da00000001 older than oldest timestamp 5ad496da00000004: Invalid argument