It's cool seeing all the engineering that goes into optimizing performance of diffs. I'm working on a FreeCAD workbench that generates diffs on CAD model trees[0], and although my bottlenecks are a bit different, I can still implement some of your optimizations down the line if needed (such as deferred syntax highlighting).

My main bottleneck is that I do a complete diff on all open + changed documents in the repository up front, because due to how document properties are stored, I won't know if the file has meaningful changes until I compute the full diff (FreeCAD may save the document, but not have anything meaningful change.)

[0] https://github.com/eblanshey/HistoryWorkbench