What are the performance implications of polymodels versus normal Bigtable models in Google App Engine?

What produces the best performance in regular Google App Engine use, a polymodel or a normal "Bigtable" model? The polymodel, effectively, creates a column in the parent table called "class" which provides the inheritance tracking. Whereas a normal Bigtable, inherited from a parent class, creates a new and separate data structure, without the ability to query the parent and find all children of all subtyped classes.