For the Revit rendering tool, the rendering process involves the simulation of light interactions with materials. As a result, render performance is significantly affected by lighting calculations. When preparing to render an image, consider the following.
Render time is directly proportional to the number of lights in the scene. In general, the rendering engine requires more time to render more lights. Consider turning off lights that are not required for the rendered image. See Control the Lights.
In general, an interior view takes longer to render than an exterior view. An exterior view with no natural light (that is, at nighttime) that shows many interior lights turned on takes a long time to render.
Revit uses area light sources to produce more realistic images. However, area shadows are expensive to compute. If you select Advanced when defining a custom render quality, instead of Simplified, shadows are realistic, but render time increases. (In the Render Quality Settings dialog, for Light and Material Accuracy, select Advanced.) See Define a Custom Render Quality.
When you use section boxes to limit the geometry being rendered, you can significantly reduce the amount of time required to render an image. (See Define the View Area to Render.) You can also use light groups to turn off lighting fixtures, thus reducing the number of lights that will impact the rendered image. (See About Light Groups. Remember, however, that lights that are not within the view can still have a significant impact on the quality of the rendered image.) Section boxes exclude lights that are clipped. When planned carefully and with forethought, the combined use of section boxes and light groups can greatly reduce the amount of time required to render an image.