It looks like you are hitting a CPU thermal limit, or perhaps your frame rate is locked to 60.
To check, turn off all power to your system, and unplug it from the wall. Then remove the CPU fan connector from the CPU_FAN header and place it back in the header. Also, make sure there are no obstacles than can interfere with the fan such as wires.
Also, make sure fortnite is set to higher than 60 FPS in the in-game settings.