That is not the lowest hanging fruit (where I would put trivial changes like inlining functions or switching an of-the-shelf library by a faster alternative). B

That is not the lowest hanging fruit (where I would put trivial changes like inlining functions or switching an of-the-shelf library by a faster alternative). But it is medium-low hanging fruit. Getting measurements that help you identify where the bottlenecks really are can take time. Often these have to be retrofitted into existing complex system, when the project is already underway. Sometimes the only way forward is replacing the current way of doing things with a more efficient way of doing things. This requires understanding the current way well (it can be the result of many teams working together for months) as well as coming up with a new design. Both of these things take time. Finally, there's bug hunting, where you get into rabbit holes. "There's a slowdown when we're on this particular step. You investigate for days, until you realize that the slowdown only happens when the user mouse is near a section of the screen. It turns out that there's a non-visible UI element that should be inactive but it isn't because it is a custom element instead of one provided by the UI library, and it is attempting to render a special effect but finds no graphical context, so it request for a new context once per frame." And one week of dev time has passed.