Nintendo of Japan has revealed that the Wii U framerate drops to 30fps if you’re playing with two Wii U GamePads. If you’re just playing with one Wii U GamePad then the framerate should be completely unaffected.
In a follow-up, Nintendo Japan has posted on Twitter that there’s a price to pay if two Gamepads are active. Frame rates drop to about 30 fps if two are active; any games that really want that extra controller won’t be quite as glass-smooth. On the upside, it’s possible to switch one off with a smooth transition; there won’t be a need for theatrics just to switch to a solo game.