Are you sure to be in performance mode? and not in quiet mode? (in my legion 5, the shortcut is fn+q, the led become red. if it is blue you are in the quiet mode, if it is white, it is the balanced mode.)
Do you have your GPU displayed (not the graphic integrated) in the task manager > performance tab ?
Finally ,you can also verify that Fortnite is launched with the GPU. You can try to switch Fortnite to high priority also in the task manager too
Ask me, if you need more information or other questions