League runs at 40 fps on MEDIUM setting while all modern AAA games run at 100+ on the best settings

As in the title, I came back to league and this shit is even more unplayable than in the past. My fps jump between 20 and 200, staying around 40 for 99% of the time. Any idea what could be the issue? FPS stay without a change no matter the settings, the only slight difference is that it runs better on fullscreen than borderless.