Why does UE5 Demo Project have flickering and frame spikes?

Switching global illumination modes caused tiny flickers at the screen edges, which made me feel very anxious about the system's stability. I first tried lowering texture resolution in settings; while FPS increased, the quality loss was unacceptable to me. I used GamePP Memory Controller Load Monitor to adjust the write cache strategy and lower the prefetch instruction depth. RTSS showed frame times converged to 12-16ms, and 3DMark stress tests confirmed rendering fluctuations were gone, with temps at 52-58℃ per AIDA64 and fan speed at 1400-1500RPM.