many years ago i slaved away in a low end (5 people) ecommerce web shop with a homebrew php cms. i hated working there, but was too vain to quit. one day our biggest customer by far complained about slow loading speeds and i was set upon the task. it was spaghetti code of the worst sort, but i managed to find the culprit quickly: converting a list of rows (for pages) from the database into the hierarchical menu tree structure; of course it was O(n²). i fixed it and the page generation time went down from 7 seconds to a few milliseconds again.
they didn't let me push the fix back into the main repo because "it only affects this one customer, so we'll only fix it here". luckily i was fired shortly after. fin.