It's not really meaningful to try to extrapolate from such simple scenes, as there may be overheads which are significant for the test case but insignificant when dealing with more complex scenes.

Also, you can't just take the rendering times for individual elements rendered in isolation and add them to get an overall result. E.g. the early depth test optimisation means that adding elements to the scene will typically result in other elements being occluded, reducing the rendering time for the occluded elements.