QuerySet is an extremely powerful abstraction. They are lazily evaluated only when needed. Hence, building longer QuerySet by method-chaining (a form of fluent interface) does not affect the performance.
In fact, as more filtering is applied, the result dataset shrinks. This usually reduces the memory consumption of the result.
A model manager is a convenient interface for a model to get its QuerySet object. In other words, they help you use Django's ORM to access the underlying database. In fact, managers are implemented as very thin wrappers around a QuerySet object. Notice the identical interface:
>>> Post.objects.filter(posted_by__username="a")
[<Post: a: Hello World>, <Post: a: This is Private!>]
>>> Post.objects.get_queryset().filter(posted_by__username="a") ...
