I think the issue with Mittelstadt is that he's not only not used to the pace of play the Avs play at, he's just plain not in good enough physical condition to handle it. And that is NOT a diss on Mitts, it happens with bad teams, especially bad teams with bad coaches. Players just get into bad habits both on and off the ice, and in practice, and they're just no longer in prime shape. Yes, I know, insert combine pull-up image HERE, but I think he'd be doing a lot better if he had come aboard a lot earlier.
Give him a full offseason to acclimate and I think he'll be fine. At this point of the year there's just not much practice time with the new teams, and I still say he hasn't been given great linemates for the most part. Bednar is the best coach he's ever had by far and probably the first one who's had anything remotely resembling proper structure. There will be a learning period.
Personally I've been okay with his play under the circumstances. Regardless of how he's done recently, even at his worst he's been far, far better than Johansen, and that guy had NO excuse to be out of shape, unlike Mitts.