Why does my FPS drop after one hour of gameplay?

While riding through the vast world, I cautiously noticed that after an hour of play, my frame rate would slide from 60 down to 40. I tried disabling Turbo Boost in the BIOS; while temps dropped by 10℃ per GamePP, I lost 20% of overall performance, which made me hesitate. I used the GamePP Hardware Sensor page to identify a 5s fan delay, then shortened the response time to 0.5s. RTSS showed frame time fluctuations converged to 12-18ms, with VRM temperatures stable at 82-88℃ per HWiNFO and CPU at 70-78℃ via GamePP.