Camera Tracking - Nuke 101: Professional Compositing and Visual Effects - page 445

As you can see in front of you, the bounding box, representing the part of the
image that is usable, is all the way around the image—even though a good 66%
of the image is black and unusable. You will make the reflection render faster by
telling Nuke that the black areas are not needed.
11. With ScanlineRender1 selected, insert a Crop node from the Transform toolbox.
12. Move the top and bottom crop marks in the Viewer so they are sitting on the top
and bottom edges of the actual panorama ( FIGURE 10.38 ) . This will make for a
more optimized (faster) tree.
Figure 10.38. Changing the bounding box for a more optimized tree.