First off Dez has definitely lost a step he can't get separation to save his life.
Second it looks to me like he just gets winded out there and doesn't have the tenacity to put himself in a position like he used to when Romo would just Chuck it.
For instance when Dak threw that horrible pass down the middle for an interception, Dez a few years ago would have at least broken it up. He saved Romo's but Many Times breaking up bad throws.
Thirdly as I said a few years ago if Dez keeps jumping 3 feet in the air and coming down awkwardly he's going to injure himself and he did and now he's too old to just leap up in the air and land awkwardly , he knows it, that was a huge part of his game and now it's gone.
So at this point he's just horribly overpaid and probably should be traded for a box of Twinkies and lose the salary cap hit.