1 - SH3D doesn't use the video card in Q3 and Q4, only the CPU core(s).
2 - Rendering time of 20 hours is not unheard of, depending on your computer and design.
3 - Reducing rendering time can be achieved by:
- reducing image size
- reducing number of light sources
- reducing number of complex objects like trees, plants
- making invisible all objects that will not appear in the image
- etc