Why does my FPS drop to 30 while swinging in Manhattan?

It was like watching a slideshow; at the most intense moments of swinging through the city, my frame rate would suddenly plummet from 90 to 30. I tried reducing the crowd density in the game settings, but the stuttering persisted during high-speed movement and the city just felt empty. I used the GamePP Power Plan Switcher to force High Performance mode and raised the BIOS power limit from 125W to 150W. This suppressed peak frame times from 50ms down to 16-22ms per RTSS, and CPU voltage finally stabilized at 1.32-1.38V per HWInfo64, with temps at 72-81℃ per HWInfo64.