The CPU is required to process the digitizer input, figure out what it's trying to do, communicate it to the application, process it within the application, turn that into pixels on a constantly-updating canvas, and express that canvas as a texture to the GPU.
The GPU is responsible for displaying that texture to the screen.
As a gamer, there's a similar issue in gaming: a lot of modern games are getting bottlenecked on the CPU (just like drawing on the Surface is), because drawing to the GPU requires several layers of software abstraction. This means that a lot of games (mostly MMOs lately) will lag no matter how good your video card is if you don't have a sufficient video card. In fact, up to 40% of the CPU use of some games is not spent by the game, but by the drivers that sit between the game and the GPU.
The GPU is responsible for displaying that texture to the screen.
As a gamer, there's a similar issue in gaming: a lot of modern games are getting bottlenecked on the CPU (just like drawing on the Surface is), because drawing to the GPU requires several layers of software abstraction. This means that a lot of games (mostly MMOs lately) will lag no matter how good your video card is if you don't have a sufficient video card. In fact, up to 40% of the CPU use of some games is not spent by the game, but by the drivers that sit between the game and the GPU.