I'm not sure what you are watching then. The last few games he is constantly being picked on whether zone or man. And he is completely selling out for the run which is why he is being killed in play action. I do wonder if he is selling out for the run to help make up for his lack of speed.
I try and not get caught up on looking at just a small sample of games. Over the course of this season he has made some great man coverage plays and yes he has got caught out in zone coverage several times as well. When he plays zone he always seems to know where the first down marker is and so is great at keeping receivers from getting a first down, but as soon as they run routes behind his zone he is either not deep enough or has last track of where they are.
His last three games have been his worst in coverage as a MLB, but we can't discount the previous 5.
I see absolutely no issue with his game speed, it's been very rare to see him get beat when he can pick his own angle to the runner.