Starting in 4.0.6, with the initial collection splits (when sharding a collection) being driven by the database primary, the shard takes the critical section for the collection, which blocks writes. If the collection is not empty, then the shard will call splitVector in order to determine the initial split points for the collection.

For very large collections, the splitVector call can take very long time, during which the collection will be unavailable for writes.

In order to improve this, we should rearrange the code so that the splitVector operation is run outside of the critical section.