I think the portable is gonna be Nintendo 'n Nvidia's "trial run.", to see if Nintendo can make enough systems for it to be worth it for Nvidia. If it goes well, we'll see a beefier console with the same type of chip and some type of connectivity between the two.
It will be slower than the PS4, but in the same ballpark. Meaning it'll be fast enough for the next Madden, Assassin's Creed, etc.