Plug your charger.
Happened to me yesterday
When my charger was unplugged, my FPS didnt go over 30. Tryed everything but still nothing has changed
Plugged my charger in, FPS jumped to 60.
Tryed plugging and unplugging several times, same results. I don't know if this is only in my case, maybe my laptop malfunctions when not in charging mode, but I had to suggest that.