Why does FPS plummet during army clashes in Rome?

The moment the general ordered the charge, the screen would freeze; this severe dropping of frames was a total nightmare for strategic gameplay. I first tried lowering shadow quality in-game, but it only gained 3 FPS and made the graphics look cheap, leaving me anxious. I used the GamePP Power Plan Switcher to force 'High Performance' mode and raised the fan start-up voltage to 0.7V via BIOS. My CPU frequency finally stabilized at 4.2GHz per AIDA64, and the FPS range narrowed to a smooth 72-78 per RTSS.