When it comes to framerate, what is not being rendered is arguably as important as what is being rendered.

Despite state-of-the-art hardware, it may be computationally expensive to send many primitives to be transformed, textured, and shaded only to find the primitive is not visible, possibly not even in the view frustum.  This is especially true in today’s multi-pass rendering techniques.

Any primitive completely outside the view frustum does not need to be rendered (it can be culled). 

For primitives partially in, only the visible part needs to be drawn (it is clipped).

High