This page performs okay-ish on Google Chrome, but it performs badly on Firefox, and it doesn’t perform well on slow Android phones, either. It’s crazy slow on Firefox on Android. (Scroll down a bit to see what I mean.)
This is probably Firefox’s fault.
Nonetheless, I think the spoiler plugin should set spoiler text to use
color: black; background: black instead of
filter: blur(0.5em) on browsers/platforms that handle blur slowly/poorly (Firefox, Android)