为什么这个线程方法需要900毫秒才能完成?

private void Form1_Load(object sender, System.EventArgs e)
{
    StartToFillTree();
}

private void StartToFillTree( )
{
    try
    {
        tvFolders.Nodes.Clear();
        tvFolders.Nodes.Add( new TreeNode("Loading", -1, -1) );

        Thread explorerThread = new Thread( new ThreadStart( FillTreeInAnotherThread ) );
        explorerThread.Start();
    }
    catch (Exception exc)
    {
        //Do the catch
    }
}

private void FillTreeInAnotherThread()
{
    try
    {
        CreateTree(this.tvFolders);
    }
    catch (Exception exc)
    {
        //Do the catch
    }
}


public void ClearTreeview( TreeView tv ) 
{
    tv.Nodes.Clear();
}

public void AddNodeToTreeview( TreeView tv, TreeNode node ) 
{
    tv.Nodes.Add( node );
}

public void CreateTree(TreeView treeView)
{
    try
    {
        TreeNode hoofdNode = new TreeNode( "NodeNam", 0, 0);
        AddDrivesToHoofdNode(hoofdNode); //This method takes 1 ms

        //This part takes 905 milliseconds if I can believe my profiler !!?? -----
        if( treeView.InvokeRequired )
        {
            treeView.Invoke( new ClearTreeviewDelegate( this.ClearTreeview ), new object[] { treeView } );
            treeView.Invoke( new AddNodeToTreeviewDelegate( this.AddNodeToTreeview ), new object[] { treeView, hoofdNode } );
        }
        else
        {
            treeView.Nodes.Clear();
            treeView.Nodes.Add(hoofdNode);
        }
        //End of 900ms part ?? -----


        AddFavorieteFolders(treeView);//This method takes 1 ms

    }
    catch (Exception ex)
    {
        //Do some catching
    }
}