Why does Unknown 9 drop from 60 to 30 FPS during rendering?

While stealthing through secret rooms, large-scale light rendering caused my FPS to instantly drop from 60 to 30. I first tried enabling power-saving mode in the BIOS, but while it dropped temps by 3℃, my minimum FPS tanked even further. I entered the GamePP Fan Curve Configuration, switched from Silent to Extreme mode, and locked the fan speed to 2400 RPM for >80℃. RTSS monitoring showed frame times improve from 22-40ms to 14-20ms, and HWMonitor confirmed temps stabilized at 74-80℃.