Why does the FPS plunge during combat in First Descendant?

During high-intensity dungeon missions, my screen suddenly turned into a slideshow, which is fatal in fierce combat. I initially tried lowering shadow quality in-game; while FPS increased by 5, the visuals looked terrible, leaving me anxious. I then used the GamePP Resource Scheduler to set the game's disk I/O priority to Highest. This worked wonders: average FPS recovered to 62-68 per RTSS, random I/O stabilized at 42-50MB/s per GamePP, and temps stayed within 68-75℃ per AIDA64.