An application can be slow even when you have a well-designed database and a powerful server. Sometimes the problem is not that the database cannot handle a query, but that the application keeps asking for the same data over and over again.
This is where caching comes in.
Caching allows frequently used data to be stored in a faster layer so it does not have to be retrieved or calculated from scratch every time. One of the most common tools used for this purpose is Redis.