Upsert performance is degraded when the number of docs is increase

Couchbase Lite iOS : 3.1.1

Based on the forum post, Upsert performance is degraded when the number of docs is increase, for example, "Upsert operation for 20k records took ~18 sec, when I have only 20k existing record in DB".

The PO has provided the test application and has done  time profiling which can be seen in the forum post. I have downloaded and attached the test app in this ticket.

https://www.couchbase.com/forums/t/couchbaselite-swift-ios-upsert-is-too-slow/37357