The time it takes to render an image depends on the quality of rendering. There are a couple of different qualities of rendering. Flat Shading The quickest rendering method. Some programs are able to render flat shaded objects in real time . When rendering using flat shading, the computer takes creates an average color for an object and then renders each face with different amounts of black or white added to simulate shading.
Objects look flat and unrealistic.
No textures are applied in this mode. Gourad Gourad rendering takes flat rendering one step further. The computer figures out values of a color on vertices of a face and then applies a gradient to the face to simulate shading. This mode is not much slower than flat shading and can also be rendered in real time on some of the more powerfull computers. Phong Phong rendering takes rendering yet one more step further.