Games that last 30+ minutes cause extremely low fps (Seems to only be on morde)

I have recently been noticing that when i play Mordekaiser and the game goes on for atleast 30 minutes or longer my fps completely drops i go from a solid 60+ to 40 (Which honestly isnt to bad) but after that it just keeps dropping. I played a 1 hour game last night and my fps went from 60+ at the start of the game to no higher than 15-20fps and teamfights got me at about 5-10 maybe as well as the friend i was playing with told me he was having the same issue.

I dont have the best computer in the world but i have always been able to play league with a solid 60 fps or higher. Another note im pretty sure it only happens when i play Morde.