More than the number of lines, it's the number of characters on a single line that causes the slowdown. Thankfully I don't have a real usage scenario where I ha

More than the number of lines, it's the number of characters on a single line that causes the slowdown. Thankfully I don't have a real usage scenario where I handle thousand character line files. I can edit multi-thousand line (5k+) Org documents just fine. So it's probably the major mode or some minor mode(s), or a combination.