Following the dashed line of the stock pump at 5000 rpm, the pump delivery is 15 gpm. This is the total volume at 5000 rpm, with 2.6 gpm being delivered to the box/rack. The other 12.4 gpm is being re-circulated internal of the pump. This excess flow is what creates high fluid temperatures and robs horsepower.
Analyzing the Turn One pump graph, it achieves 2.6 gpm flow control at 1300 rpm. At 5000 rpm it's pumping a total of 10 gpm, re-circulating only 7.4 gpm. That's 5 gpm less, resulting in lower fluid temperatures and less parasitic horsepower consumption.
You may ask why the production pump is designed this way. The reason is to have more than adequate flow at low engine rpm. So due to production variation, higher than average steering effort wouldn't be experienced during low-speed maneuvers.