Why does my GPU overheat and drop from 120 to 85 FPS in Space Marine 2 with a Jonsbo CR-1000MAX case?

Intense multiplayer combat in Space Marine 2 caused my Jonsbo CR-1000MAX case airflow to fail, dropping FPS from 120 to 85. I tried adding more case fans, but the effect was limited and the thermal issue persisted. I used GamePP Fan Curve Configuration to optimize the case layout, setting front fans to intake and rear fans to exhaust. GamePP Benchmark confirmed frame rates stable at 115 FPS and GPU temperature dropped to 75C.