Ben Adida discovered an inefficiency in acs_objects that was slowing things down "considerably." But this can be true of *any* schema and *any* set of queries.
Ben talked to me recently about a performance problem in ACS Objects that may well be the same one you're referencing. It wasn't really objects per se, it was the code that computes the PG tree_sortkey for the object.
The tree_sortkey in question is needed for the permissions implementation. The object model per se doesn't require it, and a different permissions model laid on top of the object model wouldn't necessarily require it ...
