How to fix GPU throttling in Onimusha 2 with Noctua NH-D15S?

Onimusha 2 combat dropped my FPS to 48. I lowered textures, but it looked bad. I used GamePP to raise GPU fan speed from 60% to 80% with smart control. GamePP Hardware Sensor Page confirmed GPU temp at 75C and frame time stabilized at 8-10ms.