Created attachment 298751 [details]
dmesg

Monitor connected via display port to radeon hd5770 card.
Mouse cursor and windows movements are choppy, monitor says that video mode are 1920x1080 at 44Hz.
$ xrandr
Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 8192 x 8192
DisplayPort-0 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 600mm x 340mm
   1920x1080     60.00*+  59.94  
   3840x2160     30.00  
   2560x1440     59.95  
   1920x1080_59.94  59.94  
   1920x1080_59.55  59.53  
   1920x1080_59.57  59.57  
   1920x1080_59.56  59.56  
   1920x1080_59.53  59.53  
   1600x900      60.00  
   1280x1024     60.02  
   1280x800      59.81  
   1280x720      60.00    59.94  
   1024x768      60.00  
   800x600       60.32  
   720x480       60.00    59.94  
   640x480       60.00    59.94  
HDMI-0 disconnected (normal left inverted right x axis y axis)
DVI-0 disconnected (normal left inverted right x axis y axis)
DVI-1 disconnected (normal left inverted right x axis y axis)

With 2560x1440 refresh rate are 27Hz (from monitor), and there are black screen while trying to set 3840x2160. I don't know whether it's regression. hdmi and dvi works as expected (at least with 1920x1080).