Why does Manor Lords have unstable FPS during city expansion?

While exploring my bustling manor, the frame rate would jump violently between 65 and 90 FPS, making the experience feel unstable. I initially tried lowering shadow quality to reduce load; while FPS increased, the depth of the image was lost, and I was unhappy with this compromise. I entered the GamePP Fan Curve Configuration Panel and forced the fan speed to 75% at 60℃. GamePP Benchmark Verification confirmed the fluctuation dropped below 2%, with core temp at 62-66℃ per HWInfo and clocks at 2200-2400MHz per GPU-Z. It hit 1800RPM