Is he not performing too well or is it just because he’s on big wages
I don't think those are the reasons of the termination. It may be caused by personal request to move or his attitude issue. We know that Atletico now has Suarez, Diego Costa won't have a full time to play there. Also, he often has a problem with his attictude, this can be a reson as well. But whatever the reason, I think there is no significant change in Atletico although Diego Costa moves to another club.