I have an adjacency list of objects (rows loaded from SQL database with the key and it's parent key) that I need to use to build an unordered tree. It's guaranteed to not have cycles.

This is taking wayyy too long (processed only ~3K out of 870K nodes in about 5 minutes). Running on my workstation Core 2 Duo with plenty of RAM.

Any ideas on how to make this faster?

public class StampHierarchy {
    private StampNode _root;
    private SortedList<int, StampNode> _keyNodeIndex;

    // takes a list of nodes and builds a tree
    // starting at _root
    private void BuildHierarchy(List<StampNode> nodes)
    {
        Stack<StampNode> processor = new Stack<StampNode>();
        _keyNodeIndex = new SortedList<int, StampNode>(nodes.Count);

        // find the root
        _root = nodes.Find(n => n.Parent == 0);

        // find children...
        processor.Push(_root);
        while (processor.Count != 0)
        {
            StampNode current = processor.Pop();

            // keep a direct link to the node via the key
            _keyNodeIndex.Add(current.Key, current);  

            // add children
            current.Children.AddRange(nodes.Where(n => n.Parent == current.Key));

            // queue the children
            foreach (StampNode child in current.Children)
            {
                processor.Push(child);
                nodes.Remove(child); // thought this might help the Where above
            }
        }
    }
}

    public class StampNode {
         // properties: int Key, int Parent, string Name, List<StampNode> Children
    }