If you are one of the unfortunate few who needs to use the RichTextBox for your text display project, I do feel sorry for you. If you can get along by just using the same font/size/style, then you won't have a performance problem. However, if you try and do much formatting of the text for coloration or style changes, you will find that this .NET control is abhorrently slow.
Finally, for ScriptUO I had to write a set of routines to quickly context color the text. If you use the methods included with the base class, you can expect a control containing 150K bytes, or about 5500 lines of text to take more than 4 MINUTES to color. Yes, MINUTES.
Since this was waaaaay too long, I found myself looking for a better way to do this, but still trying to utilize the RTB control. I finally came up with a set of routines which I could write to the control directly with Rich Text Formatted syntax, and quickly draw the control content. The 4 minutes was trimmed to just under 1 second.
I'm going to post the code I came up with in hopes that it'll help someone who's stuck using this control. Ultimately I had to dump the RTB completely since there was still performance issues during editing of such large amounts of data that require constant formatting.
So keep checking back and hopefully soon I'll have this code ready for generic deployment.
TM