About perfomance

Hello. If I choosing “dark theme” and go to, for example, on this address: Compton configuration · GitHub, my browser start slow down when scrolling.

My system:
Debian stable 10.8,
Firefox 78.8.0esr (64-bit).
WM - Xmonad,
Compton disabled (and any compiz).

If I disable “dark mode” in my account work fine.

P.S. Maybe this place wrong for bugs, but i don’t find where i can write of this issue.