Why does RDR2 4K MOD cause frame drops in urban areas?

I was thrilled with the 4K MOD, but Saint Denis became a slideshow as my CPU hit 88-94℃. I first tried adding two intake fans to the case, but the core temperature only dropped by 3℃, making the effort feel useless. I used the GamePP Fan Curve Configuration to push speeds to 1900 RPM above 75℃, utilizing a 6.5V start voltage. RivaTuner showed frame times finally converged to 14-18ms from a choppy 22-45ms, and GamePP Hardware Sensor showed temps stabilized at 74-78℃.