Why does frame fluctuation persist in Kingdom Come Deliverance II even after lowering graphics quality settings?

Frame rate fluctuated in open plains. Lowering quality sacrificed detail. GamePP BIOS Load Line Voltage Adjustment Wizard set voltage to 1.28V. CPU temp is now stable at 70-75℃ per GamePP Hardware Sensor Page, and frame rate stays above 60 FPS in light/shadow scenes.