There are two ways that you can affect the performance of GNU diff by changing the way it compares files.
Performance has more than one dimension.
These options improve one aspect of performance at the cost of another, or they improve performance in some cases while hurting it in others. The way that GNU diff determines which lines have changed always comes up with a near-minimal set of differences. Usually it is good enough for practical purposes. If the diff output is large, you might want diff to use a modified algorithm that sometimes produces a smaller set of differences. If the input files have a constant small density of changes, this option speeds up the comparisons without changing the output.