Which sharding architecture generally provides the lowest request latency but at the cost of significantly higher client-side complexity and coupling?

With a 'Smart Client', the routing logic lives directly in your application code. Because the client connects directly to the correct shard, it avoids the extra 'Network Hop' and CPU overhead of a central proxy. However, this creates a major maintenance burden: if you add a new database shard, you must re-configure and deploy *every single instance* of your application. Proxies are more popular for large teams because they decouple the application from the database topology, allowing for zero-downtime cluster changes without touching application code.