Why is Tomb Raider stuttering like a slideshow in ruins maps?

Entering ruins, my game stuttered badly. I increased page file to 16GB, but FPS dropped from 76 to 59. I used GamePP BIOS Wizard to set timings to 36-38-38-72 and voltage to 1.30V. AIDA64 confirmed latency dropped to 75-80ns, and MemTest86 showed zero errors after 6 loops. VRM temp stayed at 57-62C. The fix works perfectly.