The most AiOs have thin aluminum radiators and very simple cooler. Thats the reason why custom parts performe better than AiOs + you have much more water in a custom loop. And the water flow is higher in a custom loop. Some AIOs have only around 10 L/h while a custom loop should have minimum 35-40 L/h. The most people like to have between 60 and 100 L/h.