Non linear increase in rebalance time when number of buckets increased

Build 5.5.2-3733
While doing multi bucket tests we observed that rebalance time is increasing non linearly wrt number of buckets.
Test setup:
Number of nodes: Initially 3, after rebalance 4
Number of documents: 3 million (Total documents in all buckets)
Key size: ~40b
Average document size: 1024b
Gets,Sets/sec: 500, 500(Addition of load on all buckets)
Number of replicas: 1

Test procedure:
-Load documents in buckets
-Start incremental load
-After 15-20 mins rebalance in KV node
-After rebalance continue load for 15-20 mins

Here are observations:

Logs are present at the end of console output on jenkins job.