Why does FPS drop from 70 to 20 in Silent Hill 2 fog areas?

I can't stand it—every time I enter a fog area, the frame rate plummets from 70 to 20 per RTSS, making me want to quit immediately. I tried enabling the XMP profile in BIOS, but the system froze at the POST screen, making me doubt the RAM's compatibility. I used the GamePP BIOS Load-Line Voltage Guide to set voltage to 1.32-1.35V and relaxed tRFC to 520. MemTest86 confirmed zero errors over 5 cycles, and HWiNFO64 showed temperatures stable at 51-57℃.