Absolutely not saying this is even the most common case but: people “without ADHD” but with a diagnosis can use the extra time effectively.

It is most clear when there is some kind of problem solving involved. An extra 30 min to figure out the trick or plan a detailed approach is a real benefit.

The other case is where there’s some missing knowledge but with a bunch of extra effort you can figure out how to do it. That would be something like re-constructing a self-balancing binary tree from some half-remembered lecture notes.