It's not a question of moral virtue. There's no moral virtue in cooking with spices, either. Watching a movie at 2x may make it worse, but it's not immoral.
> That's unhelpful black-and-white thinking.
Unhelpful to what? What's the goal here? Simply getting to the end of the movie? They put middles in movies for a reason—they're a load-bearing part of the experience. Why sit through a warped & incomplete version of a mediocre movie when you could just turn it off & watch the complete version of something consistently good? Of course, if you're fast-forwarding through pieces of everything, this doesn't work, but that's a red flag in and of itself, and it should prompt you to ask yourself why you can't sit through a whole movie. Do they all have unacceptably slow pacing? Or is there a problem with your expectations?
I should point out, also, that I don't mean this in a black-and-white sense. I'm not saying that fast-forwarding through one scene one time is some sort of mark against you. But if you find yourself doing this consistently, how can you be sure that the problem isn't on your end? The point of holding yourself to standards isn't finger-wagging; it's to provide an objective lens to view yourself through. How would you know if you did have a problem with your attention span, if not by monitoring yourself for the coping strategies which a person with a low attention span would use?