Why does my FPS drop from 60 to 45 over time?

While playing Neverness to Everness, I noticed a stealthy decay where FPS slid from 60 to 45. Thermal Probe showed fin temps at 55-62℃. I tried adding two intake fans to the chassis, but core temps only dropped by 2℃, a negligible result. I used the GamePP Fan Curve Configuration Panel to force fans to 2000 RPM above 75℃, with a 6V start per GamePP. After two hours, core temps remained stable at 76-80℃ per HWMonitor, and fan speeds stayed at 1400-1900 RPM per GamePP to verify