Why does S.T.A.L.K.E.R. 2 drop from 60 to 45 FPS in ruins?

The FPS dropped to 45 while exploring ruins. Lowering textures didn't help. I used GamePP Memory Controller Load Monitor to boost VRAM from 2250MHz to 2350MHz. 3DMark confirmed bandwidth rose to 750GB/s, and the frame rate stabilized at 100 FPS, verifying the fix worked.