If we were to extend the first graph a bit more to the right, the linear improvement would quickly trend downwards into negative latencies, a sure sign that it's not the right answer. But if linear is impossible, then super-linear is just as impossible. The line the author describes as such is clearly asymptotic.
If we were discussing overall latency, and not just queue delay, then an asymptotic decrease towards zero could correspond to a super-linear increase in throughput capacity. But that's not what's happening here, either: because average latency is bounded below at one second, total throughput can never exceed c. There is no super-linearity to be found here.
If we were discussing overall latency, and not just queue delay, then an asymptotic decrease towards zero could correspond to a super-linear increase in throughput capacity. But that's not what's happening here, either: because average latency is bounded below at one second, total throughput can never exceed c. There is no super-linearity to be found here.