How to search several words in a RichEditBox with over 20,000 lines

I have a speed performance issues. I would like to ask how would you guys do this.

I am using Visual Basic Net 2005 and am making a Programming Editor for our Healthcare software.

I use a RichTextBox to load a text file which will result in over 20,000 lines of text. Then i have to search each line for 10 or more specfic words. The result of the search will then appear in a tree-structure List control.

I use Richtextbox.find function to search for the words but it seems to be very slow. How would you improve the speed issues?