I have tested implementing some improvements. Checking if we are in cell we already highlighted before clearing the interface and rehighlighting does improve the performance massively as long as output is hidden. I'll work on cleaning that up and checking if the output should be redrawn or if it even is in the visible range next.

Great!

I remember this would be a slightly tricky thing in the code, so I had opted to do the simple thing first and then keep see what I needed to optimize. As it turned out, everything felt very snappy for me so I thought we were, surprisingly, actually fine with the naive implementation. How did you notice / how apparent are the performance issues?

To be fair, I am working with a software engineerical mess of a 2500 line script with a cell with 350 lines so this is probably an edge case. As you can see in the video, scrolling gets laggy and choppy, with it keeping on scrolling after releasing j/k. As I said, with hidden output this does not happen when we only rehighlight lines if the cell span changes. I'm currently a little busy but I'll open a draft pr at the end of the week.