"Your brain. It’s so … fast."

[Permanus]

The entire post-Crisis as we know it could simply be a dream of Superman's, with Superboy-Prime's punch really a metaphor for the other side of Superman's brain telling him to wake up and go to work already. 

You know, much as everyone except Patrick Duffy hates the "it-was-all-a-dream" getaway clause, that one actually makes sense in a comic-booky way. Superman could wake up in the middle of his team-up with Superboy-Prime, who might be slapping him in order to get him to wake up.