How to stop FPS drops in AC Valhalla siege battles?

During a massive siege in England, my frame rate plummeted from 75 to 40, which is lethal in combat. I first tried enabling Power Saving mode in Windows; while it dropped temps by 5℃, the minimum FPS fell even further, leaving me feeling helpless. I then accessed the GamePP Fan Curve Configuration Panel and forced the fan speed to 100% at 80℃. After this, core temperatures stabilized at 76-82℃ per RTSS, and frame times converged to 10-16ms per FrameView. My gameplay is finally smooth again.