When raster engraving, the laser sweeps back and forth in horizontal rows, going from the left-most pixel to the right-most. If there is empty space in the middle, the laser will shut off as necessary, but the lens will continue moving at the same speed. This results in lost time while the machine is running, but not actually engraving anything.
For example, think of engraving this image:
There is a large amount of area where the lens will have to pass over, but not actually fire. In the following image, these areas are highlighted in grey:
Because the image contains numerous separate sections, it can be adjusted to engrave different sections as different jobs, eliminating much of this dead space.
Not every project needs to be optimized. The most important consideration is the amount of time saved by optimizing versus the amount of time it takes to set up an optimized project.
The best way to optimize a single image is to split it up into multiple images. This works best when there are distinct areas of the image (such as the continents in the world map example). Spiting an image by breaking apart connected parts is not recommended, since the laser might not align the two pieces perfectly.
It is rarely possible to get a perfectly optimized image. Simply try to eliminate as much empty space as possible.
There are several ways to optimize an image:
Splitting the image and using color mapping will have the same results - which one is best is a matter of personal preference and the nature of the image.