Why does Gray Zone Warfare drop to 30 FPS in rainforest zones?

While sneaking through the rainforest, my FPS would tank from 80 to 30, which is fatal during a firefight. I tried disabling all shadow details in-game; it gained me 10 FPS, but the stutters from VRAM overflow remained, making me very anxious. I used the GamePP DLSS/FSR Image Sampling Calibrator to set sharpening to 65 and render resolution to 95%. VRAM usage dropped from 14.2-15.8GB to 12.1-13.4GB per GPU-Z, and frame times converged to 12-16ms per RTSS, while temps hit 68-75°C per AIDA64.