Currently it uses nsIAccessible::GetPreviousSibling() what was never performant and iterates children from the start. After the bug 573955 this problem should go away but any way it uses nsIAccessible to traverse the tree. We should get rid this. I think it makes sense to combine it with nsAccesisble::GetLevelInternal() method if there's a nice way at least they have the same logic.
Alex, this should be fast now right?
(In reply to comment #1)
> Alex, this should be fast now right?
right, even more than bug states, node_child_of relation for ARIA trees is exposed based on group information and thus smart. Fixed by bug 575576.