Why does FPS drop from 90 to 50 in Splinter Cell Remake shadows?

While sneaking through dark corridors, the frame rate plummets from 90 to 50 FPS whenever dynamic shadows cover the area. I tried Extreme Performance Mode in BIOS, but unstable voltage caused crashes every fifteen minutes. I used the GamePP Resource Scheduler to switch the memory mapping mode to Low-Latency Priority. RTSS monitoring showed peak response time shortened from 3.8s to 1.9s, while AIDA64 showed data exchange latency at 110-140ns. HWiNFO64 confirmed temps at 55-61℃ with a stable 6800MHz via BIOS Monitor.