[Zope] Slicing a lazy sequence

Hi, I'm looking at a performance issue with a site. Part of the problem seems to be the time taken to load all the attributes to construct a large number of brain objects. I think I remember reading somewhere once that if you slice a sequence of catalog results as follows: rs = portal_catalog.searchResults(portal_type='my_type') return rs[10:30] or even <dtml-in "portal_catalog.searchResults(portal_type='my_type')" start=10 size=20> that this "undoes" the advantage of the lazy sequence, as it has to iterate over all the items in the sequence to produce the slice. Is this correct or have I just imagined it - can anyone confirm?! And if I haven't imagined it, how can I do this more efficiently? Thanks, Miles