Dual 4K slowness due to xorg threading concept?

[Thomas Wunder]

Hi all,

Recently I got a new workstation consisting of these components:
Two 4K Monitors (DP 1.2 at 60Hz), i7-5820K (not yet overclocked), MSI X99A-
based mainboard, Radeon R7 250.
This is the software I'm running on it (Debian Jessie):
Linux 3.16.0 Debian build 4, fglrx 14.9, xorg 7.7, (KDE 4.12)

Problem:
"Graphics stuff" seems to work smoothly on lower resolutions (e.g. 2x 
2560x1600). On high resolution (2x 3840x2160) still everything 
(moving/resizing windows, playing videos, scrolling through lists, text views 
or consoles, opening menus) goes smoothly but only as long as single windows 
are really small (and as long as there are not too many of them). As soon as a 
window becomes half the size of a screen or even full screen, operations 
mentioned before become horribly slow (K-Menu takes up to 8 secs to even open 
up) and CPU usage of the Xorg process is vastly increasing (to about 100%).
I don't really have an idea of what's going wrong here. There are no apparent 
errors etc.  The only thing that seems really suspect to me is the 100% CPU 
which I would attribute to a (multi-)threading issue...
I'd really appreciate if you could give me a hand and point me to the right 
direction to get things working.

Cheers
 Thomas
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