A GPU is possibly more energy efficient given a much more complex scene, animation, effects, etc. than what you would draw with CPU, and much faster. But in case there is no GPU, one must definitely go without any complexities, heavy draw calls, etc. Kind of obvious, but may get forgotten. I certainly don't know what's best in this case, unless I have some time on my own to experiment and see. One reason custom, but fixed hardware tends to get better software over the time, is that developers learn how to use it best (for example: game consoles/systems/arcades, classic computers... whether they have GPU, or not (in the distant past)).