Answer
A query that cannot be routed by the partition key is sent to every shard and the results merged. This is a scatter-gather query, and its latency is set by the slowest shard.
Also accepted: scatter gather, fan-out for scatter-gather.
Why
With 100 shards, even a 1% chance of a slow shard per request makes most scatter-gather queries slow. Choose keys so the frequent queries hit one shard.