During chunk migration, the donor shard will record all document deletions and transfer them to the recipient, regardless of whether the deletions are relevant to the chunk being migrated or not.

This causes significant load on both shards.

The reason for this is that the document being deleted is no longer available on the donor shard.

One solution would be that in the logOp call we should pass the shard key of the document being deleted so it can be filtered out.