A directory has no data attribute. But, as an index, it has instead three other file attributes: index root, index allocation, and bitmap. The index is stored in the nodes of a B+ tree in the following manner:
The B+ tree structure (which is used in HPFS too), when built in a balanced way, is far more efficient than a linear structure to perform a file name lookup in a folder containing a large number of files.
Although the duplication of the stream of the indexed attribute in an index entry can cost some time, it is worthy because you can browse an index without actually opening all the indexed files (FAT and HPFS do that, too).
In a directory, the three file attributes: index root, index allocation, and bitmap are named "$I30", and a directory is just an Index of file attributes whose type is 30. But NTFS has been thought as a database filesystem, and it can actually create indexes based on any file attribute that is always resident. E.g., you could create a new file attribute labeled "author name", and sort your files according to that criteria.