title: Restrict the initial queryset to objects in pagination in django-restframework
body: <p>I am defining a <code>ModelViewSet</code> using <code>django-rest-framework</code>.<br>
  I need to override the default queryset to perform some processing on the queryset objects before rendering the response. </p>
  
  <p>This process is time-expensive so I would like to execute it only on the objects that will be actually available to the consumer due to the paginated response, <strong>instead</strong> of applying this process to <strong>ALL</strong> the objects and applying pagination <strong>AFTER</strong> finishing my processing, which I can notice (correct me if I'm wrong) is the default behavior in DRF.</p>
  
  <hr>
  
  <p>In short what I need is:</p>
  
  <p>If the default queryset is <code>1000 objects</code>, but the pagination is restricted to <code>25 objects per page</code>, I want to apply my process only those <code>25 objects</code>. Please note there is no other constraints for reducing the final amount of objects other than pagination.</p>
  
  <p>Is there a way to do this? <br>
  Is overriding the default queryset a bad idea in this case?</p>
  
  <p>Thanks!</p>