So Mario kart 8 runs on the Wii U at 720p 30fps (not quite sure about 30fps, please correct if wrong) and on the Switch it runs docked 1080p 60fps. 1080p takes roughly twice the amount of power to render, so combining that with 60fps versus 30fps it is roughly four times as powerful as a Wii U when docked.
Feel free to debunk/correct; this is all idle speculation.
Edit: so apparently the frame rate is the same as Wii U, but the resolution is bumped to 1080p. Still roughly double the power of Wii U while docked, and similar when undocked, and still not counting other graphical enhancements.