title: CoreData memory and performance managment
body: <p>I'm building an iPhone App where the user gets to add data to a CoreData database and generate reports of them later on. 
  the report generation will be repeated and will have to be displayed frequently. </p>
  
  <p>My question is:
  should I copy the CoreData entities (might be in hundreds) to an array and process them or should I use nested fetch request to retrieve the data. I'm worried about either the app being too slow or too memory consuming.</p>