Mordekaiser massive performance loss. As literally unplayable as it gets

I don't have the best rig and I know Morde is a shit champion, but I enjoy playing him from time to time. By standard, I get 120 FPS in ultra settings and sometimes go down to 90 in teamfights, but whenever Mordekaiser is in the game, whether I am playing him or he's just on the teams, FPS drops significantly. Early game it isn't so bad, only 1-2 fps, but by around 20 minutes, it goes from 120 to 30, and at 30 minutes, it freezes frequently. PLEASE get somebody on this, it's suffering to play as or against.