I've just plugged together my new workstation (
running the following software:
The KDE desktop seems to work smooth at this resolution (2x3840x2160, side by side) when there are litte to no windows open, however as soon as there are two maximized windows (e.g. file browser, web browser (no web sites loaded) or email client) the desktop manager reacts unbelievably slow. For instance if I try to move or minimize window, or even just try to open the K-menu it takes ages (many seconds) for those operations to be carried out. Interestingly the mouse pointer moves smoothly while a moving window prepares for its next "jump".
Watching the top command to see how heavily the CPU is being utilized reveals that one core is at 100% as soon as there's more than one fullscreen (but more or less inactive) window.
Thanks in advance!
PS: Attached is my xorg.config (screen arrangemet is done through this KDE "Display Configuration") and my current Xorg.0.log