Ask HN: Why do editors still struggle with long lines?

I've noticed that editors tend to struggle much more with long lines than long files (many lines), even in 2025 with a recent computer.

For example, I was recently working with a csv with 50 columns, and each value was a bit long.

Things got sluggish at the end of the lines in Helix [0][1][2].

(side note: diffing this kind of files is also a pain, too, but that's another story)

It isn't the first time I notice this issue (the first time I tried diffing some minified Javascript with 100k+ long lines). So I guess it might be due to syntax highlighting on top of text encoding [3].

(To partially answer my own question: this performance metric is probably not tested nor optimised for in the first place. And also because not every edit/movement uses async rendering).

[0]: https://github.com/helix-editor/helix/issues/4513

[1]: https://github.com/microsoft/vscode/issues/55605

[2]: https://emacs.stackexchange.com/questions/598/how-do-i-prevent-extremely-long-lines-making-emacs-slow

[3]: https://stackoverflow.com/questions/7388916/why-are-text-editors-slow-when-editing-very-long-lines