To achieve sub-millisecond query latency, which is significantly faster than what Cloud SQL for PostgreSQL can typically offer, you should consider using in-memory solutions. Memorystore is Google Cloud's fully managed in-memory data store service, compatible with Redis and Memcached, which is designed to provide sub-millisecond data access. Configuring Memorystore and modifying your application to offload queries to it would be the most effective way to achieve the desired performance improvement. Options A (Firestore) and B (Bigtable) are not optimized for sub-millisecond latency for all types of queries, and option C (Cloud SQL read replicas) would not provide the necessary latency reduction as it still relies on disk-based storage.