Do you have several options:

• @ittus's and @Andrey Shipilov's approach: store result in cache system like Memcache or redis. You can check github.com/niwinz/django-redis

• @Anonymous's approach: use some sort of materialized view (depends on your db), and then create a unmanaged model in django to query it.

• Create new models/fields for aggregate data and figure up it nightly though a django custom comand or a celery task.

Conclusions:

• Your question should be close because is a primarily opinion-based question, like this one Django - what is best practice - Calculating field values.

Notice:

• If you need real time calculation then you can overwrite save method on models to keep data or just connect a post_save signal.