Rules are user-defined actions that require a server to perform on behalf of the user. Typical examples of rules are: displaying a notification when a message is received from a specific person, and automatically moving messages into a specific folder according to the content of the message. Typically, rules have very little effect on overall server performance until most users have more than 10 rules set up.
For views, the server must store and keep track of the indexes that make up a view. Although a cache is used to store the most recently used indexes, a user might notice a small decrease in performance when opening a seldom-used view.