Why does MGS Delta drop from 90 to 40 FPS in jungle scenes?

Sneaking through the rainforest was a pain when the frame rate suddenly tanked from 90 to 40 FPS, creating a severe sense of discontinuity. I tried manually locking the memory frequency in BIOS, but it caused two memory checksum errors during boot, which was incredibly frustrating. I then used the GamePP Power Plan Switcher to enable Ultimate Performance mode. Memory frequency stabilized at 3200MHz per CPU-Z, and RAM temps stayed at 40-50℃