Page "Rendering (computer graphics)" Paragraph 55

Due to the large number of calculations, a work in progress is usually only rendered in detail appropriate to the portion of the work being developed at a given time, so in the initial stages of modeling, wireframe and ray casting may be used, even where the target output is ray tracing with radiosity.

It is also common to render only parts of the scene at high detail, and to remove objects that are not important to what is currently being developed.