No matter how much we push to the GPU, the CPU is still needs to stay available and free from other work. The GPU cannot do anything without the CPU sending all the data to the GPU. If the CPU gets interrupted, the data flow stops, causing frame rate stutters or uneven frame flow.
The fastest video card on the planet is worthless without data to work with. If the CPU cannot keep the data flowing to the video card, then that world class video card might as well be a VGA adapter.
As far as the percentage breakdown between the CPU and GPU goes, it can vary greatly, depending on what is happening, in any given frame.