The code inside ShardRegistry::_lookup() blindly applies the latest connection string for each shard to the ShardRegistryData being generated. Most of the time, these connection strings will be the same as what's already in the ShardRegistryData, meaning that the work of integrating them is unnecessary. It should be possible to short-circuit (continue) prior to this work, if the returnData ShardRegistryData already associates this connection string with the correct shard.