QueryableBackupMode will cause the `_sizeStorer` member to not be instantiated: https://github.com/mongodb/mongo/blob/master/src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.cpp#L269

When the `_sizeStorer` does not exist, the WiredTigerRecordStore ctor table scans to count up how many documents exist: https://github.com/mongodb/mongo/blob/master/src/mongo/db/storage/wiredtiger/wiredtiger_record_store.cpp#L855

This is not ideal. It seems that the size storer should be able to read data off disk while in QueryableBackupMode to initialize its state, while also avoiding any writes to disk as data should not be changing.