Reducing the number of iterations or comparisons by a constant number may make a performance difference, depending on what gyrations you have to go through to make the reduction, but you're not changing the order of complexity. A 3-N solution is the same order as a 3/2-N solution, is the same as a single-pass (N) solution. They're all O(n). This is an important concept.

If you can reduce the order of a solution, your solution will scale better. We commonly look for O(n log n) solutions to replace O(n²) solutions, so that working with large amounts of data doesn't make our app bog down. If you merely change by a constant factor (as we're talking about in this case), you may see a constant-factor improvement at any size, but you won't alleviate any scaling problems. You're in the realm of micro-optimization.

Walking through the list is not going to be a significant portion of the computation, compared to the comparisons and math being done on the variables of interest. And it's really not worth trying to take the elements two at a time, because that ends up being a very inefficient way to walk through the list, compared to Perl's built-in for.

If our OP were to translate the algorithm into Inline::C, the reduced number of comparisons might compete well with three List::Util calls, with the difference being some constant factor on any size list.