Why does my Splinter Cell stutter during stealth?

I felt the immersion shatter when my character lagged in the factory. I tried cleaning temp files, but it failed. I used GamePP NVMe Queue Depth to set priority to Real-time. RTSS showed frame intervals dropping from 14.3-18.1ms to 9.1-10.4ms, and AIDA64 confirmed stable 48-55℃ temps.