The original quote is not about things generally taking a long time, but about time complexity i.e. differences between things like O(n), O(n log n) and O(n^2). For example, if the way you fixed that performance bug reduced the time by a fixed multiple then you didn't affect the time complexity, even if it's now much faster.
Maybe you already understood that but your bug description makes it sound like complexity wasn't necessarily the issue.
My case was about time complexity from calling the DB 10x times more than necessary. Even though these things are all fast now, as an engineer, you still need to know the techniques to avoid the slow bits
Although my example was only O(n^2), the constant of each n being over the wire to the DB blew the runtime up to a significant number. So even an n-squared algorithm with small data sets can kill your app. We changed it to be O(n) over the wire to make it reasonable
Maybe you already understood that but your bug description makes it sound like complexity wasn't necessarily the issue.