Text censor editing UI is really slow with large text

[rugk]

STR

1. Paste a large text part as an "post received" in a request.
2. Open the dialog to edit the text.
3. Now hover, click on some words to unredact.

What happens

1. The hovering animation is really slow. You can hover over a word and it takes multiple seconds until the "grey background color" fades away.
2. I can then click on it and it stays like that (white/transparent background) and move to the next word. That part is okay.
3. However, clicking before the hovering animation begins does not show the new result directly. It works, but also takes several seconds.

This really makes redacting a pain. Seems there is some bad performance issue, maybe it uses JS (synchronously) for the animation or iterates the whole text?

What should happen

Instantly show the hovering animation and the unredacted results should also be applied immediately.

Further notes

• Also the loading screen that appears when you open the dialog takes multiple seconds, which is not fast IMHO.
• When you redact a new word, the same happens. In my case the whole website UI even froze (the pointer icon as a mouse cursor stayed as it is even if the mouse is moved).

System

Firefox 84 Linux/Fedora 33