lag on kayn

when kayn does his circle lighting thing i lag super hard and for the rest of the kayn fight i have 6 fps for the rest of the game after it when i have 60 for the entire game before that one atrack, and it makes it really hard to complete the hardest difficulty stage, i have all my shit on the lowest settings and i use a low resolution and i have my nvidia optimized but i still lag every time on kayn's lighting thing :/ maybe you guys know more than me but i dont know how i can make my potato run better