As far as the performance is concerned, it is better to reduve no. of joins in the query by doing a de-normalization. How can we do a de-normalization?

Is using a formula of Account field in Contact a sense of de-normalization so that we do not use Account object while writing a query on the Contact object? Are formula fields stored as a data internally and updated as and when the source data gets updated? If no, are roll-up summary fields also calculated at runtime rather than storing them internally?