Mousepad slowness due to long line

Mousepad version 0.5.9 on Arch Linux.

How to consistently reproduce:

1. Visit this page: [https://web.archive.org/web/20210120214743/https://www.publico.pt/2001/07/14/jornal/daniel-faria-o-rapaz-raro-159820]
2. Press ^A to select all the page contents, followed by ^C to copy them to clipboard.
3. Configure Mousepad to use Word Wrap (tick box off the Document menu bar).
4. Press ^V to paste contents of clipboard into Mousepad.
5. Scroll to the bottom in Mousepad. The issue is that it is very slow to scroll. On my 4 years old PC one of the cores maxes out while scrolling.

Note: I tried the above in Leafpad, Geany and Kate with word wrap enabled. I also tried in LibreOffice Writer and Gedit. None has this issue. A probable cause of the issue is a long line, meaning many characters before the line feed.