The option _secondaryThrottle when set to true will GLE w:2 after every single document being copied/cloned.

For cases of very small documents (aka very large number of documents in chunk range) when there are also inserts going on in the from shard into this range, this can be a lot slower (relative to unthrottled migration) than for a collection with fewer documents in a chunk.

Maybe the waitForReplication() should be every N documents where N is some fraction of maxDocs in chunk (so say, waitForReplication() every 1/100th of chunk or 1/1000th). Otherwise the delay it introduces is too unpredictable.