During recent testing balancer enhancements I found that initially sharding a collection with a small chunk size will significantly increase the duration to complete sharding collection relative to sharding an empty collection and loading the data with the same chunk size.

Test Scenario - MongoDB 5.0
1:

• 4TB Collection Size
• 32KB Doc Size
• 1MB Chunk Size
• Shard Existing Unsharded Collection with sh.shardCollection

2:

• Set 1MB Chunk Size
• Shard Empty Collection with sh.shardCollection
• Insert Documents
• 32KB Doc Size
• 1MB Chunk Size

Scenario 1 will take much longer than Scenario 2