 <p>You might need to consider using a different text box control.</p> <p>Daniel Grunwald has written the Wpf text editor for SharpDevelop completely from scratch. It is called AvalonEdit and a good article is on codeproject:</p> <p><a href="http://www.codeproject.com/KB/edit/AvalonEdit.aspx" rel="nofollow noreferrer">http://www.codeproject.com/KB/edit/AvalonEdit.aspx</a></p> <p>It seems that he has done optimizations for large files.</p>