Deeply/truly threaded forums are definitely computationally intense. You have to leverage something to recreate the necessary tree from it's database structure

Deeply/truly threaded forums are definitely computationally intense. You have to leverage something to recreate the necessary tree from it's database structure (preorder tree traversal, adjacency lists, etc) while applying ACL methods, view transformations, etc on the dataset. This is much less of a problem nowadays though (compared to when forum software was "in vogue").