Why does Hellblade II FPS swing between 40 and 20 FPS?

During the immersive story, the frame rate swings between 40 and 20 FPS, which completely ruins the cinematic atmosphere. I tried lowering the resolution, but while I got 5 more FPS, the stutters from power instability remained. I used the GamePP Power Plan Switcher to lock the system to Maximum Performance and disabled CPU C-states in BIOS. RTSS showed frame times converged to 18-24ms, AIDA64 confirmed CPU temps at 72-78℃, and HWInfo showed VRM temps between 88-95℃.