Why does Kuro no Kiseki have erratic FPS in busy town scenes?

While shopping in a town scene, my frame rate jumped erratically between 45 and 60 FPS, which felt super anxious at 4K. I tried limiting the max frame rate to reduce heat, but while the temp dropped 3℃, the minimum FPS fell even lower, which was discouraging. I then used the GamePP Fan Curve Configuration to lower the start threshold to 50℃ and trigger full speed at 75℃. RTSS showed frame times stabilized at 8-14ms, and AIDA64 confirmed the CPU temperature remained stable at 68-75℃.