WPF Multiline TextBox for large content
If you do not expect much more than ten-thousands of search results in your application, a TextBlock control or readonly multiline TextBox will suffice by far.
The TextBox class has an AppendText() method which should be fast enough for you.
If you need text highlighting / formatting then maybe you want to use RichTextBox.
If you have really large content, then unfortunately all the WPF textbox and similar controls are very slow. See this question. You could use AvalonEdit as a replacement.