I'm currently using KDE Plasma on Archlinux and Firefox windows are very laggy when maximized. I have a 4K screen and running Firefox at full 4K causes scrolling to be very choppy and makes YouTube almost unresponsive. If I make the window smaller (about a quarter of the screen or the size of typical 1080p), it speeds up and works normally. I've tried the nightly build, Icecat, and Waterfox and none of them have made any difference. I use a AMD Radeon R9 Fury X GPU and have the latest version of xf86-video-amdgpu installed and have hardware acceleration enabled in Firefox settings. EDIT: I've now tried Firefox in Cinnamon Desktop as well as KDE Plasma and they are both equally laggy.