xps 怎么添加水印，我使用如下代码添加水印太慢了。。请问还有什么方法可以让他变快？

Question

  void AddWatermark(string filename)
        {
            // Open original XPS document
            XpsDocument xpsOld = new XpsDocument(filename, FileAccess.Read);
            FixedDocumentSequence seqOld = xpsOld.GetFixedDocumentSequence();

            // Create new XPS document
            string path = Path.GetFileName(filename);
            Package container = Package.Open("new_" + path, FileMode.Create);
            XpsDocumentWriter writer = XpsDocument.CreateXpsDocumentWriter(new XpsDocument(container));

            // Needed for writing multiple pages
            SerializerWriterCollator vxpsd = writer.CreateVisualsCollator();

            int pageno = 1;
            vxpsd.BeginBatchWrite();
            foreach (DocumentReference r in seqOld.References)
            {
                FixedDocument d = r.GetDocument(false);

                // Walk through each page
                foreach (PageContent pc in d.Pages)
                {
                    FixedPage fixedPage = pc.GetPageRoot(false);
                    fixedPage.Background = new SolidColorBrush(System.Windows.Media.Color.FromArgb(1, 1, 1, 1));
                    double width = fixedPage.Width;
                    double height = fixedPage.Height;
                    Size sz = new Size(width, height);

                    // Convert to WPF Visual
                    fixedPage.Measure(sz);
                    fixedPage.Arrange(new Rect(new Point(), sz));
                    fixedPage.UpdateLayout();
                    //将文档背景设置为透明色
                 
                    ContainerVisual newpage = new ContainerVisual();
                    newpage.Children.Add(fixedPage);
                    newpage.Children.Add(CreateWatermark(width, height, "Confidential (" + pageno + ")"));

                    pageno++;

                    // Write out modified page
                    vxpsd.Write(newpage);
                }
            }

            vxpsd.EndBatchWrite();

            container.Close();
            xpsOld.Close();
        }

 Visual CreateWatermark(double width, double height, string message)
        {
            DrawingVisual watermark = new DrawingVisual();

            using (DrawingContext ctx = watermark.RenderOpen())
            {
                FormattedText text = new FormattedText(message,
                  System.Globalization.CultureInfo.CurrentCulture,
                  FlowDirection.LeftToRight,
                  new Typeface("Times New Roman"), 96 * 1.5,
                  new SolidColorBrush(Color.FromScRgb(0.3f, 1f, 0f, 0f))
                );

                // Rotate transform, keep center
                {
                    double diag = Math.Sqrt(width * width + height * height);
                    double cX = width / 2;
                    double cY = height / 2;
                    double sin = height / diag;
                    double cos = width / diag;
                    double dx = (cX * (1.0 - cos)) + (cY * sin);
                    double dy = (cY * (1.0 - cos)) - (cX * sin);

                    Transform mat = new MatrixTransform(cos, sin, -sin, cos, dx, dy);

                    ctx.PushTransform(mat);
                }

                // Centerlize
                double x = (width - text.Width) / 2;
                double y = (height - text.Height) / 2;
                ctx.DrawText(text, new Point(x, y));
                ctx.Pop();
            }

            return watermark;
        }