What Is Xorg
Well, your comment was not actual answer but gave me a hint. I dug deeper into man radeon and (debian wiki is rather enigmatic or outdated: still recommends EXA for performance tunning), looked at Xorg.0.log and found the reason: Xorg auto detection loads EXA acceleration method for pre-TAHITI GPUs (mine is ARUBA).
And - indeed - EXA is slow. So, the solution is custom /etc/X11/xorg.conf.d/20-radeon.conf which loads glamor acceleration method: Section 'Device' Identifier 'Radeon' Driver 'radeon' Option 'AccelMethod' 'glamor' Option 'DRI' '3' Option 'TearFree' 'on' EndSection NOTE: I've found some articles that DRI3 requires Xorg version 1.17 or higher and ati/radeon driver 7.6 or higher so don't be surprised if it doesn't work. Now Xorg is using no more than 2% CPU. Problem solved.