I just read this article about optimizing NetNewsWire for the iPhone in NetNewsWire for the iPhone. Definitely looking forward to seeing the performance improvements in the next release. It's great to see a developer who knows when it's the right time to tweak and how to do so.
tl;dr: When operating over a small subset of a large dataset use an ORM such as Core Data, when operating over a large subset of a large dataset use direct SQL access.
There are also couple of useful links in there.
Core Data isn't an ORM, even if EOF originally was.