Why does my FPS swing wildly between 144 and 82 in Apex Legends?

In the final circle, my FPS swung wildly between 144 and 82, making it nearly impossible to pre-aim targets accurately. I tried lowering shadow quality in-game, which gained me 10fps but didn't stop the frequency jumping, leaving me feeling anxious. I used the GamePP Power Plan Switcher to lock the system in High Performance mode and disabled CPU core parking. CPU frequency stayed at 4.8GHz via HWInfo, frame times converged to 6-8ms per RTSS, and fan speed locked at 2100RPM per GamePP while temps hit 74-78℃ per HWMonitor.