RichTextBox highlighting methods

Hi! I am working at a syntax highlighter project for school and I need some suggestions.

Firstly I tried to do that using the .SelectionColor property of the richtextbox, but I found that it is somehow slow and , when typing fast, the blue selection is visible. It is the same even if I stop the repaint of the richtextbox .

The second time , I tried to use rich text format , which is fast , but it is difficult to use and I encounter some errors , like colors change suddenly.

Should I try to learn more about those methods or is there another way to do that ?

I mention that the text which I want to highlight can have more than 1500 lines.

Thank you !