You seem to be confusing "knows how to say 'oh of enn squared'" with "the loop gets slow when I iterate over a lot of things". One is generally a product of education, the other, merely experience.

The idea that a thing can only be learned in a classroom is perhaps in the top ten most pernicious ideas in the modern world, and probably one of the more surprising ones to show up in that list. You do not need special courses to discover that your code runs slowly, and as I've seen often enough, having had those special courses does not confer immunity against writing slow code.

Now, I am also a believer that formal education has its place, and if you are going to get a formal education in computer science, big-O analysis absolutely must be part of it or you are literally missing out on an entire rather important sub-discipline. But the idea that it's some sort of touchstone between Good and Bad programmers is just ludicrous nonsense. Slow code is slow. There are abundant tools that can be used to figure out why. If you can't work out why your O(n^3) loop is running slowly after a couple of years of practicing the art, you don't need formal education, you need a different job.