Mordekaiser causing FPS drop

I was playing an ARAM match and the enemy team had a Mordekaiser. Any time that I had visual contact with him or he was out of the Fog of War (not sure which caused it) I had an FPS drop from 60 to 35-45. As soon as he died or went into the Fog of War, my FPS would return to 60. Attached is the Match History just in case it is an interaction between champs.