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