IndexingXPathNavigator enables lazy or eager indexing of any XML store (XmlDocument, XPathDocument or any other IXPathNavigable store) thus providing alternative way to select nodes directly from built index table and not searhing the tree. This allows drastically decrease selection time on preindexed selections.
For more info/updates/comments/etc go http://www.tkachenko.com/dotnet/IndexingXPathNavigator.html.