Balancing performance after adding a shard

can a single-core performance of primary config server be a bottleneck of chunk migration after adding an additional shard?

I have a 4-shard (3 nodes each, plus 3 config server) 4.4.1 cluster. Each node has about 3TB compressed (zstd) data, ~15TB uncompressed. I have recently added a new (5th) shard due to DB growth. After a few hours I calculated that balancing will take long weeks or even months. I also found that mongod process on primary config server uses about 1 full CPU core (on a 4-core) system and seems to be a bottleneck since I/O (including network) and CPU usage on other nodes is rather low. Is the balancer process single-threaded or it just doesn’t want to use the whole CPU? Can I speed it up by adding more CPU cores?
Is there any other way to speed the balancing up?

Without that I may have disk space issues on existing nodes since I won’t be able to release disk space fast enough.