IO bound might mean latency but not throughput, so you can up concurrency and add batching, both of which require more concurrent requests in flight to hit your real limit. IO bound might also really mean contention for latches on the database, and different types of requests might hit different tables. Basically, I see people say they're IO bound long before they're at the limit of a single disk, so obviously they are not IO bound. Modern drives are absurdly fast. If everyone were really IO bound, we'd need 1/1000 the hardware we needed 10-15 years ago.