Why does PCCOOLER RZ820 throttle Civ VII CPU frequency from 5.2GHz to 3.8GHz during late-game turns?

Late-game Civ VII turns with 50+ cities spike CPU load to 95%, causing the PCCOOLER RZ820 to throttle. I tried maxing fans to 2000 RPM, but the noise was unbearable and only dropped temps by 3C. I used GamePP Fan Curve Configuration to set 70C to 1500 RPM, 80C to 100%, and a low-load limit of 600 RPM. AIDA64 confirmed core temps stabilized at 78-82C and frequency held at 5.0-5.2GHz, restoring turn times to 2 seconds.