Not sure exactly what the widget/UI element is called, because as you can see I've never used Adwaita and don't even know the terminology.

This slider here is very slow to update in the web preview. Don't know if it's an implementation issue, or a problem with the slider itself, but I would assume the latter because the value changing box thing on row 3 updates just fine when I move the slider. Because it updates faster than the slider, I can say with confidence it's probably just a graphical issue. Sorry I couldn't be of more help.
Firefox 84.0.2, Arch Linux.