Why does AC Valhalla lag after one hour of gameplay?

Running across the fields of England, the once-smooth visuals began to drop frames after an hour, ruining the experience. I tried lowering the environmental detail settings, which helped the GPU but left the CPU overheating, a compromise I found unacceptable. I used the GamePP Fan Curve Configuration to lock the pump at 100% and boosted the radiator fan response. Frame rates returned to a stable 55-60 FPS per RTSS, and CPU temperature stabilized at 70-76℃ per HWInfo.