Why are turn transitions so slow in the late game of Civ VII?

Early game turns were instant, but late-game calculations for thousands of AI units turned every turn into an agonizing wait. I tried enabling Windows Game Mode; CPU usage dropped, but turn calculations still took over 20 seconds. I then used GamePP BIOS Load-Line Voltage Guide to raise voltage from 1.25V to 1.35V and activated the 6000MHz XMP profile. AIDA64 measured read bandwidth increasing to 62-68GB/s, and HWMonitor showed temperatures at 52-58℃.