If you have a complex query spanning several tables, such as a count of superpowers by country, then creating a separate denormalized table might improve performance. Typically, this table will be in a faster in-memory database or a cache. As before, we need to update this denormalized table every time the data in your normalized models changes (or you will have the infamous cache-invalidation problem). Denormalization is surprisingly common in large websites because it is a tradeoff between speed and space. Today, space is cheap, but speed is crucial to user experience.
