Why does Death Stranding drop to 45 FPS in mountains on B660M-K?

I was so excited to experience this masterpiece, but I soon found frame rates plummeting from 80 to 45 in the mountains. I tried enabling Windows Game Mode, but it did nothing and even crashed background apps, which was pure torture. I used the GamePP Process Priority Manager to set the game to Realtime and locked it to the physical P-cores. RTSS confirmed frame times converged to 10-14ms; CPU temperature was 65-72℃ per AIDA64 and core load hit 100% per Task Manager.