In the diagonal region, the throughput can rise because it is capped only by the memory bandwidth. Until the throughput reaches its peak, the application can keep doing operations on its data because the bandwidth can keep up with providing the data. Once the horizontal region starts, the throughput has peaked because the bandwidth has maxed out. At this point, the throughput is capped by how fast the processor can execute the operations.
In the diagonal region, the throughput can rise because it is capped only by the memory bandwidth. Until the throughput reaches its peak, the application can keep doing operations on its data because the bandwidth can keep up with providing the data. Once the horizontal region starts, the throughput has peaked because the bandwidth has maxed out. At this point, the throughput is capped by how fast the processor can execute the operations.