I had a similar experience when I learned about factoring (not sure what age). I got a little obsessed with finding prime numbers by hand, and eventually figured out that I only had to search for factors up to the square root. I distinctly remember that as the first time I realized how powerful a simple mathematical insight can be to speeding up a process.

Now, of course, I use mathematical insight to speed up processes occupationally; I guess that experience stuck with me.