Shard rebalance time increased from 3 minutes to 150+ minutes on RC3 build

So with custom toybuild 18464 and 7.6.0-2090.. 
we are seeing some  regression with shard based rebalance test like :

http://showfast.sc.couchbase.com/#/timeline/Linux/secondary/rebalance/Plasma 
Logs for 7.6.0-2071 : 

https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-aether-3933/172.23.110.53.zip{}
**
https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-aether-3933/172.23.110.54.zip

https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-aether-3933/172.23.110.55.zip
{}https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-aether-3933/172.23.110.56.zip

https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-aether-3933/172.23.110.71.zip

https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-aether-3933/172.23.110.72.zip

https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-aether-3933/172.23.110.73.zip

https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-aether-3933/cpuprofile.prof

https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-aether-3933/memprofile.prof

Logs for 7.6.0-18464:

https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-aether-3940/172.23.110.53.zip
https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-aether-3940/172.23.110.54.zip

https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-aether-3940/172.23.110.55.zip

https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-aether-3940/172.23.110.56.zip

https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-aether-3940/172.23.110.71.zip

https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-aether-3940/172.23.110.72.zip

https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-aether-3940/172.23.110.73.zip

https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-aether-3940/cpuprofile.prof

https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-aether-3940/memprofile.prof