windows store app RichTextBlock book app

Question

I want to build a book app, a RichTextblock with many RichTextblockOverflows, I create the overflows in code-behind, and add them to the stackpanel in a scrollviewer. I tested my app and found that it finished creating the overflows quickly, but after that it will block for a while(which is unbearable) to show the overflows in UI. I registered the LayoutUpdated event of the stackpanel and found that it would be triggered a long time after overflows were created.

How can I improve the performance? Thanks

                while (true)
                {
                    // for existing columns just set their width
                    var overflow = CreateOverflow(columnWidth, margin, lastOverflow);
                    lastOverflow = overflow;

                    overflow.Measure(availableSize);
                    _layoutPanel.Children.Add(overflow);

                    if (!overflow.HasOverflowContent)
                    {
                        overflow.UpdateLayout();
                    }

                    if (!overflow.HasOverflowContent)
                    {
                        _layoutPanel.LayoutUpdated += _layoutPanel_LayoutUpdated;
                        break;
                    }
                }