Long Rebalancing Time after Adding Shards

I run a mongodb sharded cluster v4.4.6-ent. The data size is about 80G. Previously the cluster have 2 shards and after adding 2 shards (now 4 shards), the cluster takes more than 3 days (up to now) to reblance data, which significantly increased the online service 90th latency.

Execute: db.adminCommand( { balancerStatus: 1 } ):

> db.adminCommand( { balancerStatus: 1 } )
{ mode: 'full',
  inBalancerRound: true,
  numBalancerRounds: 321397,
  ok: 1,

It seems that the cluster is still working on reblancing data. Looking at the cluster CPU usage for the past days:

The CPU usage can be divided into two phases:

  1. Moving chunks at 8/8 for 10 hours
  2. (Not sure) Rebalancing data/index in the cluster?

So I have several questions:

  1. What does the cluster do after moving chunks?
  2. How long will it take to balance data? Or how can I get the current progress (the above command only show that it’s still in balancer round)?
  3. How to minimize the reblancing impact on online service after adding new shards?