The mental ray® rendering engine uses complicated algorithms to produce a photorealistic image from a 3D view of a building model. The amount of time required to generate the rendered image varies depending on many factors, such as the numbers of model elements and artificial lights, the complexity of the materials, and the size or resolution of the image. Furthermore, the interplay of other factors can affect render performance. For example, reflections, refractions, and soft shadows can increase render time.
Ultimately, render performance is a balance between the quality of the resulting image and the resources (time, computing power) that can be devoted to the effort. Low quality images are generally quick to produce, while high quality images can require significantly more time.
Before rendering an image, consider whether you need a high quality image or a draft quality image. In general, start by rendering a draft quality image to see the results of the initial settings. Then refine materials, lights, and other settings to improve the image. As you get closer to the desired result, you can use the low or medium quality setting to produce a more realistic image. Use the high quality setting to produce a final image only when you are sure that the material render appearances and the render settings will give the desired result.