Why does Expeditions: Rome drop to 20 FPS in campaign?

I can't stand it; every time I hit a large campaign screen, my FPS tanks from 60 to 20, making me want to reboot instantly. I first tried the system's 'High Performance' power mode, but CPU temps hit 92℃ and the drops stayed, which was infuriating. I then used the GamePP Fan Curve Configuration to force fan speed to 85% load above 70℃. AIDA64 showed max temps dropped to 76-81℃, and HWiNFO64 confirmed a stable CPU temperature of 68-75℃. Now the game actually feels playable!