The slowmo vs realtime is irrelevant. Every foul is evaluated in slowmo. It is a clear foul - no questions about that... They are spending the time to assess if it is a red card or not. The fact that he does touch the ball probably makes them conclude with a yellow, which I agree with, but the way Shaw follows through with his studs and hits high on the shin makes it a clear foul. In general, getting to the ball doesn't make everthing OK. For example, even if you hit the ball clearly, but your foot is showing studs, with great force and high on the oppositions leg - it will be a red even if you are going through the ball. It's a bigger question if this is a "clear and obvious" error enough by the ref for them to go that far back in time.
I agree that the Cavani situation should result in a red and the disallowed goal by Maguire does look soft. The defender gets rewarded for backing into Maguire and not jumping. It's the arm in the upper back from Maguire that's the issue.