Why does my FPS drop from 120 to 40 in Banishers combat?

During massive exorcism battles, my frame rate would plummet from 120 to 40, which was absolutely breaking. I tried setting the BIOS fans to full speed, but the noise was like a hairdryer in my ear and the temperature stayed around 90℃. I used the GamePP Fan Curve Configuration Panel to set a stepped gradient between 70-90℃ and a full-speed trigger at 95℃. Core temperatures stabilized at 82-87℃ per HWInfo, FPS hit 85-105 via RTSS, and fans started at 1200 RPM per SpeedFan.