QoS approaches like this are very well studied and very useful, but still have limits. The queuing itself consumes resources and throughput-domain requests that

QoS approaches like this are very well studied and very useful, but still have limits. The queuing itself consumes resources and throughput-domain requests that are already in flight will still contend with latency-domain requests (unless you want to implement preemption which is usually very difficult in a system not originally designed for it and also leads to further resource waste). If you want really low latency you have to be completely isolated from the throughput domain.