Why does Forza Horizon 5 drop from 110 to 45 FPS in highlands?

Driving through the highlands, my FPS suddenly tanked from 110 to 45, making the car feel sluggish and unresponsive. I tried lowering PCIe link speed to 3.0 in BIOS, but read/write speeds halved and loading took 6s longer. I used GamePP Fan Curve Configuration Panel to move the trigger from 55℃ to 40℃ and enabled max airflow. SSD temperature was suppressed to 58-64℃ per AIDA64, and FPS hit 88-105 per RTSS.