In reply to Sally G..

If you are on a shared hosting server that is overloaded, you might experience slowness in pages. This advice might urge you to install a plugin like WP Super Cache, which will cache pages for users that are not logged in.

An object cache module here might notice if Memcached or other object caches are available. You can than be adviced to install a plugin that will make use of this. On a website that is data heavy and uses lots of database requests you might notice a speed improvement. This advice I have mixed feelings about, because object caches can sometimes cause problems by showing old data.