Meh, I bought that explanation for a while, that it was just a question of the barca system but I don't buy it anymore.
And like I said, physical attributes do matter, they have a part to play. If he's too slow, and gets exposed then it seems logical to criticize him for it. It's not like it's a rare sight to see him act inexperienced anyway. He does make other types of mistakes as well. Those terrible slide tackles for instance...it's not just a question about which team he plays in. Even if there's some truth to it of course.
He should be a better attacker of the ball. It's not a question about attackers dribbling him but they manage to create a dangerous situation almost every single time. They just have to turn, or change direction and they can get a shot or put a cross in. Either he gets quicker, or gets better at stopping them.
You even said so himself, he's not quick enough to keep up. Is that a mistake? Well, he's obviously slow, he can't help it. It's perhaps not a tactical mistake but it's not good defending.
Kompany isn't really that quick either, he still doesn't look like a fool whenever a defender tries to challenge him. Even if it doesn't happen as often as with Pique.
I like that you are trying to defend him though. I really do. No sarcasm.