Syntax Highlighting

Question

Can someone take a look at http://www.c-sharpcorner.com/Code/2003/June/ColorSyntaxEditor.asp and tell my why after about 800 lines or so the performance drops like a rock? I've written my own version of a syntax highlighter using the RichTextBox and no matter what I tried I ran into the same thing. The code to highlight the text only looks at the current line...I don't understand why it goes so slow as the size gets bigger. If I take out the code to highlight, I can make it as big as I want without any problems. Please - someone explain this to me.