Currently the RichEditBox (or even the RichEditBlock, or even the TextBlock) are not virtualized. As a result with a moderate amount of runs in the box it becomes almost unusable slow near 4000-5000 lines.
The request is, create a virtualized RichEditBox that is capable of handling large files.
There's considerable request for this type of control. Here are a couple github issues that would be resolved if there were a virtualized textbox or richeditbox available: