We want to know if balancer is efficiency enough to balance data when we add a new empty shard to a cluster

The detail of the test

mongodb version : 3.0.5
OS : centos 6

We try to use tag aware shard tag to pre-allocate the chunk in shard2 and try to insert around 10 millions of records to it.

– The disk iops is around 10 during the insertion

Then we remove the shard tag in shard2 and add it back to shard3 and let the balancer to move the 10 millions of records from shard2 to shard3

– The disk iops is around 100 and more. And the whole machine is slow down

After the test, we are worrying that the entire cluster performance may drop if we try to add a empty shard to a cluster because a certain portion system resource is used to balance the data

May I know if the improvement of balancer performance would be part of the roadmap ?