Not meant to look scary at all, in fact given its a explicit generalization it should make what's going on in both Peachs and Xenas cases easier to understand. If you replace xenas values with a, b, c, and d you would then expand the expression and get the same one I have given, for peach she is literally using this expression and just plugging in the numbers.
The upshot of what i am saying is that different methods of doing multiplication take different amounts of time. We know this because such methods are literally just Algorithms in the computational sense, and algorithms are given time complexities which are just ways of finding out how fast a computer can do a specific computation using a specific algorithm.
Both expressions are easy to memorize, fundamentally they are both grade school math.
In the case of Peach and Xena they have memorized ac10^n + 10^(n/2)[ad + bc] + bd even if they've never seen it before given this is literally what you learn to compute n-digit multiplication problems in grade school. The Algorithm is commonly called the grade-school multiplication algorithm. In theirs and most peoples cases they know it implicitly.
ac10^n + 10^(n/2)[(a + b)(c + d) - ac - bd] + bd is computationally slightly faster than the grade-school method because it involves one less multiplication. If you did memorize it you would compute multiplication problems slightly faster than someone else using the grade-school method whom also processes information at the same rate as you do. And, this speed up is more and more obvious the larger you make n, so while you maybe only be a sec faster than someone when computing small n-digit numbers when n is made to be huge you would finish minutes before them.
