Why does MGS V lag during stealth on Soyo B365M?

Sneaking through Afghan mountains, I felt my character's turn stutter. I tried clearing temp files, but background processes still ate 14.2-16.8GB of RAM. Using GamePP Resource Scheduler, I set priority to Real-time. AIDA64 showed frame intervals drop from 8.3-12.1ms to 5.1-6.4ms, and CPU temp stabilized at 58-63°C.