When I'm playing Mordekaiser, after about 20 minutes or so I start to see large fps drops.
I start off the game with about 50-60fps and within 20-30 minutes I have as low as 20fps.
It's not specific to a skin as I have tried Pentakill, King of Clubs, and the default Mordekaiser.
It also is not specific to me as other players have been complaining about these low fps drops too.
This is only since the rework, I'm surprised this hasn't been hotfixed.
Before his rework, this was never a problem.