I got my Mordo from the arena the other day and started fighting with him yesterday. Right away I noticed when I do a combo and then try to block, there's a consistent lag. This happens against ANY fighter, in any arena/map. I've used him in all the catalyst daily arenas, Dr. Strange arena, and normal Guardians event with the same results. If the AI is aggressive, I get rolled, if it stalls a bit before it retaliates, I can usually recover and block. You'll see from the video below though, the lag is consistently there, but only after a combo attack.
ipad Mini-4 latest iOS, latest build of mcoc (happens on my iphone 7+ too)
https://www.youtube.com/watch?v=7ROoTYK-39c