Why does Lumen GI cause FPS to drop from 60 to 20?

The Lumen visuals were breathtaking, but the frame rate plummeting from 60 to 20 FPS was devastating. I tried lowering shadow quality, but the scene lost all its depth and the CPU stayed hot, which was a waste of time. I used GamePP Fan Curve Configuration to shorten response time to 0.1s and set speed to 100% above 75℃. This suppressed temperatures to 70-76℃ per AIDA64 and recovered the frame rate to 55 FPS per RTSS, with voltage holding at 1.28V per HWInfo.