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Why is my R7 250 Eyefinity so slow on dual Display 3840x2160 using fglrx 14.9?

Question asked by tomi_wunder on Sep 16, 2015


I've just plugged together my new workstation (

  • i7-5820K,
  • X99-based Mainboard,
  • 16GB of DDR4-3000 (running @2133 since the board not yet overclocked),
  • R7 250 Eyefinity 4,
  • *Two* Asus PB287q (via DP 1.2 3840x2160@60Hz),...)

running the following software:

  • Debian Jessie -> Kernel 3.16.0-4-amd64,
  • fglrx 1:14.9+ga14.201-2,
  • xorg 1:7.7+7 and
  • KDE 4.12,...


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.



  • Might this be related to some hardware misconfiguration?
  • Is this old R7 250 too weak for 2x 4k?
  • If it's not due to a hardware issue, might this behavior be related to some configuration problem or due some missing acceleration enabled etc.??
  • What do you suggest me to do now?


Thanks in advance!

Best! Thomas


PS: Attached is my xorg.config (screen arrangemet is done through this KDE "Display Configuration") and my current Xorg.0.log