The check involves scanning the supporting index to get the recordIds and checking that shard key isn't constant. The latter can be done without extracting the shard key value from every index key value and compare every pair of pervious and current value, i.e. we can skip extracting and comparing as long as more than one shard key value has been seen. The check was only necessary back when we use to exclude recordIds with the same shard key value from the correlation coefficient calculation in SERVER-74772. Skipping this might significantly reduce the latency of the monotonicity check.