While in my most recent game playing as Karthus I experienced what at first felt like lag. Champions would move slowly and then jump to locations as would Gragas's barrels. In addition to this though my cool downs slowed down a lot. For instance I used Karthus's ult at 17:33 in the game. It had a cool down of 170/180 seconds however when we surrendered at 24:51 my ult still had six seconds left on the cool down. The cool downs would constantly tick down, just each second last much longer than it should have. This was most noticable on the longer Karthus ult (which I got to cast twice in a 25 minute game :/ ) but was applicable to all of the cool downs greatly reducing my dps.