For what it’s worth, scrolling pages with a lot of blurred text (e.g. Spider and Web Invisiclues Hints) performs pretty poorly with Firefox on my (admittedly long in the tooth) laptop.
It’s not bad enough to bother me, particularly since few threads have all that much spoiler text, but I can imagine it would be worse for someone browsing on a mobile device.
An easy fix would be to change the css for .spoiler-blurred :
filter: blur(0.5em);
background: black;
color: black;