Why does FPS drop after an hour in World War Z: Aftermath?

While exploring the wasteland, I noticed my frame rate slowly sliding from 110 to 75 after an hour, which was incredibly obvious. I first tried setting the fan to full speed in BIOS; although the temperature dropped 4℃, the piercing noise made my whole room vibrate. I decided to use the GamePP Fan Curve Configuration to change the 70-85℃ range to a step-up gradient and locked the peak at 1500 RPM. Now, core temperature is stabilized at 74-78℃ per AIDA64, CPU frequency is 4.2GHz per RTSS, and memory stays at 58-63℃ per 3DMark.