Big O is a useful tool, but it doesn't directly translate to performance. Understanding how systems work is a lot more useful and important if you really care about optimization and performance.

This is something I learned pretty early on as a professional developer. I got a computer science degree and was taught data structures, algorithms, and big O. In my first job, I came across a small piece of Java code that was being run a lot that had a small list being searched each time. I figured converting to a HashSet would be faster, but in testing it was actually slower. I forget the exact details, but I learned to test my assumptions about performance and not to just blindly change things.

My degree, for the most part, did not prepare me for working with large, interconnected systems. The only course that came close was an elective called "concurrent programming" which was really just "algorithms, but parallel".