After upgrading ReplicaSet(PSS) from 4.4 → 5.0 → 6.0, Slow Query increased significantly and when I checked Slow Log, I found that schemaLock time was the cause.
Most of Slow Query is planSummary: IDHACK but schemaLock time is long.
30,000 collections in DB.
I know that it is recommended to have maximum 10,000 collections in one ReplicatSet.
If have many collections, can the waiting time to acquire schemaLock be long?