Replica item count lagging active in Magma insert test

Note: Initially opened on performance variation between, that has been addressed, but now tracking the issue where replica item count does not reach active
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In Magma insert only tests, we see high performance variation. 

http://172.23.123.237/#/timeline/Linux/hidd/S0/all

In the latest runs with build 7.1.0-1558, the throughput changed from 122K to 226K.

http://cbmonitor.sc.couchbase.com/reports/html/?snapshot=rhea_710-1558_access_key_prefix_e3f5&&label=high_insert_rate&snapshot=rhea_710-1558_access_key_prefix_3994&label=low_insert_rate

In the run having higher throughput, replica sync rate can't catch up.

  There are more sync write flushes after a certain point.

  Sarath Lakshman

Please take a look. Is there a way we can change checkpoint settings? It looks like the runs can go to different modes (or code paths), even with the same build.