A known Wii hacker, Marcan, has apparently taken the Wii U apart to figure out its exact specs, which Nintendo has thus far not shared with the public. Many developers have come out and said, that it's painfully slow. Well - according to Marcan, the clock speed of the CPU is roughly 1/3 of the 360s/PS3s CPU. What exactly that means for developers and gamers however, I'll leave to more adequate commenters. It's apparently less bad than it implies, but certainly not good either.
It's quite telling however, that we're getting the Wii U's specs from a hacker, rather than from Nintendo. Seems like something's fishy. PR nightmare really.