The issue of heat exchanger cooling efficiency

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I have a question regarding heat exchangers that I’d like to ask everyone: Given a constant heat exchange area of the heat exchanger, as well as constant flow rates of the hot medium and its initial temperature when it enters the heat exchanger, the requirement is to reduce the temperature of the hot medium from 85 degrees Celsius to 70 degrees Celsius, with the lowest temperature remaining above the highest temperature of the cold medium. Logically, with a constant pipe diameter, the greater the flow rate of the cooling medium entering the heat exchanger, the faster the cooling rate should be. However, in practice, when the flow control valve is adjusted, the cooling rate is the same whether the cooling water valve is fully open (300 t/h) or at 60% of its capacity (200 t/h, as determined through process operation experiments). The time it takes for the heat medium to drop from 85 degrees Celsius to 70 degrees Celsius is almost the same. Why is this? (The heat medium is circulated.) I would be very grateful if everyone could help answer this question! Is it a problem with the heat transfer drive?