Why does Assassin's Creed Shadows cause CPU thermal throttling?

I was infiltrating the complex buildings of ancient Kyoto when the frame rate suddenly plummeted from 85 to 42 FPS, totally killing my stealth immersion. I first tried setting the pump to full speed in the BIOS; while the peak temp dropped by 2℃, the high-frequency whine was unbearable and didn't stop the stutters. I then used the GamePP Fan Curve Configuration Panel to bind fan speed to core temperature, triggering high speed at 65℃. Finally, CPU temperature stabilized at 68-74℃ per AIDA64, and frame time dropped to 11-16ms per RTSS.