Cost of the "is operator" (and right place to post)

Hi,
first of all, I am not quite sure if this is the right place for such a question. But I will try anyway.
Assume the following  very simple structure:
An element can have subelements. These subelements only differ by the class the derive from. This class only differs by the interfaces it implements. So assume I want to get a list of all people, I simply collect all elements which implements the IPersonName Interface. If I want a list of all "things" I could gather all elements implementing the "IName" Interface, which is more general then the IPersonName Interface. (You get the idea, a case of strict interface based programming)...
So, but now, in the pre alpha stages of my solution, I start thinking about costs. If I will have to run over the whole list of children every time I want to get the Persons and try to check every element, this MAY be very expensive. On the other hand, if I start to "buffer" this result, I have ...