Once I made a Ruby program noticably slow by using "array += items" instead of "array.push *items" in a loop, making it O(n^2) instead of O(n). Once we noted that doubling the input size quadrupled the runtime, I found the bug. (I was a Ruby newbie and didn't know that Ruby's "array += items" is just syntactic sugar for "array = array + items".)
I've used slow web pages that show a lot of items or data. One way to make them slow is O(n^2) DOM manipulation (e.g., inside a loop over items in a page, O(n)-searching the page for something).
Knowing complexity theory isn't the only way to find performance problems, and sometimes isn't even relevant, but it sure is helpful.
Oh shoot. I used array = [array, other_array].flatten to get around the += slowness. Way faster on lists of tens of thsouands that I was throwing around. Totally forgot push. (Granted, my solution was developed on Sunday morning with a hard deadline of Monday morning and I had been working hard all week. )
I've used slow web pages that show a lot of items or data. One way to make them slow is O(n^2) DOM manipulation (e.g., inside a loop over items in a page, O(n)-searching the page for something).
Knowing complexity theory isn't the only way to find performance problems, and sometimes isn't even relevant, but it sure is helpful.