Why does RDR2 drop to 15 FPS during intense combat scenes?

During the most intense shootouts, my screen suddenly turned into a slideshow, with FPS plummeting from 60 to 15. It was unplayable. I tried disabling all high-resolution textures, but the game looked terrible and the drops still happened. I used GamePP NVMe Queue Depth Configuration to lock the queue depth at 64 and disabled write cache flush in Windows. RTSS showed frame generation time dropped from a 50ms peak to 16-22ms. CrystalDiskMark measured write speeds at 400-600 MB/s, and HWMonitor recorded temps at 52-61℃.