Newly restarted nodes in a replica set suffer from a cold cache, which can make secondary reads directed to such nodes experience longer latencies.
Making use of the hidden replica set configuration flag is one technique to hide such "warming" nodes from secondary reads.
Today, we only use mirrored reads in the server to keep secondaries somewhat warm in order to improve primary hand-off. It should be possible to use mirrored reads to also expedite the warming of such hidden nodes, as an optional feature. The idea would be that we could mirror, say, 100% of reads on a primary to any nodes that are currently healthy and hidden. It would be left to an external mechanism to decide when to turn this feature on, and when to reconfigure the replica set to add or remove the hidden property from nodes.
Credit to max.hirschhorn@mongodb.com for this idea!