It all depends on the situation. MS SQL Indexed views are faster than a normal view or query but indexed views can not be used in a mirrored database invironment (MS SQL). A view in any kind of a loop will cause serious slowdown because the view is repopulated each time it is called in the loop. Same as a query.

Views are slower because the database has to calculate them BEFORE they are used to join to other tables and BEFORE the where clauses are applied. The optimizer then joins the data together along indexes as best it can, just as it does with non-view elements, and then the entire query is run.