Anybody who's read DC Comics long enough knows that Kryptonians were much more powerful Pre-COIE when the Multiverse was still in effect than Post-COIE in the single DCU. What hasn't really been discussed is *why* this is so.
I believe the change has less to do with the Kryptonians themselves than it does with the very nature of the DCU compared to the old DC Multiverse. Quite simply: the DCU's physics can't support the outlandish feats of power that the Pre-COIE Kryptonians (and other beings) were capable of.
Consider this: before and during COIE, both Kal-L (the Superman of Earth-Two) and Superboy Prime had Pre-COIE levels of power. They could move planets with their bare hands and fly from Earth across intergalactic space on their own power within a matter of minutes. Upon their entry into the Post-COIE DCU in INFINITE CRISIS, however, only Superboy Prime exhibited such powers. Kal-L was only on par with the Post-COIE Superman of Earth-One (whose power level was far diminished from that of his Silver/Bronze Age counterpart).
This principle extends not only to Kryptonians, but to all superpowered beings in the Post-COIE DCU. Pre-COIE, the likes of Wonder Woman, Captain Marvel, and the Martian Manhunter had powers on par with the Silver/Bronze Age Superman's, but Post-COIE, these characters' powers are on par with the diminished Post-COIE Superman's.
Moreover, when the Pre-COIE Silver/Bronze Age Kara Zor-El Supergirl visited the Post-COIE DCU in Peter David's MANY HAPPY RETURNS storyline, she was unable to move the Earth as she believed she could have done in her own universe.
All of this seems to indicate that the nature of the Post-COIE DCU is different from that of the Pre-COIE Multiverse, and it simply will not support the kind of extreme feats seen before COIE. Even Pre-COIE Kryptonians and similar superbeings will experience a diminishment of their power upon entering the DCU; their powers will adjust down to the rules of that universe.
Which makes Superboy-Prime's retention of his Pre-COIE power level in INFINITE CRISIS an anomaly.
Another anomaly: at the end of INFINITE CRISIS, how were the two Supermen able to fly from Earth to Krypton's orbit so quickly on their own power? The Post-COIE Superman has never had such an easy time flying through space. He could reach the moon without too much trouble, but he'd need a superliminal spacecraft or other transportation if he were to travel outside of Earth's solar system. Kryptonians can't fly between solar systems and galaxies on their own power in the Post-COIE DCU, especially not within the timeframe of minutes or hours.
Buried Alien (The Fastest Post Alive!)