Currently the mongos/mongod load the metadata lazily - only when they access a collection, they will load the chunks metadata for that specific collection.
For large collections with tens/hundreds of thousands of chunks, loading the metadata may take some time. There are some situations that loading the metadata proactively may help. For example, after a mongos is restarted and accepts the application traffic, the operations would be pending until the chunk metadata for the collection (s) is loaded. This would increase the latency for the operations.
It would be nice if we can provide the ability (e.g. an admin command) to load the metadata (for all the collections), so that it can be run after a mongos is started, and before this mongos is used for application traffic.