How to speed up MongoDB chunk moving between shards
Things like how many shards we have, which database are sharded, and the configuration setting of your shard cluster. But one of the most important pieces of information that the config servers hold, is the mapping of the chunks to shards.

By default MongoDB takes ChunkSize=64 MB


By default , MongoDB takes 64 megabytes as the default chunk size. That means that if a chunk is about 64 megabytes, or within 64 megatons range , it will be split.

1MB&lt;=chunkSize&lt;=1024MB


We can define a chunk size between the values of one megabyte and 1024 and one gigabyte. The chunk size is configurable during run time. So if we decide to change a chunk size, we can easily do so. But before we go in changing your chunk size.

for further your ref here and here