It's a much more trivial example, but highlights the point well I think - we have pages in the app I work on that would respond in ~100ms, but might have a single sentence on them that takes another 100ms to generate because of the complex data relationships involved in figuring out what that sentence needs to say. The 'request handler' might be 20 lines of code, with a 50 line util function to generate that line of text. No armchair architect will ever take into account things like that, but the end result is a page that is just a bit more personalised to the user and therefore improves their experience.
In an app of any real size, I imagine there are anywhere from hundreds to many thousands of tiny little details like this that all together drastically increase the amount of power needed to run a service.