Problem with performance with Composite and dri

[RafałMużyło]

Relevant portion of xorg.conf:
Section "Device"
        Identifier  "open-radeon"
        Driver      "radeon"
        Option          "EnablePageFlip" "true"
        Option          "AccelDFS" "true"
        Option          "VGAAccess" "off"
        Option          "DDCMode" "true"
        Option          "AccelMethod" "EXA"
EndSection
(dri and composite - enabled)
My problem: program - gvbam (gtk frontend to vbam - on sourceforge)
With fglrx, I get good framerate with both cairo and opengl output.
With this driver:
when dri is enabled:
  with cairo: system seems to freeze (I'm not talking about framerate, server
itself gets unresponsive)
  with opengl: framerate is very low
when dri gets disabled by running second server on display 1:
  with cairo: framerate is very low (but doesn't seems to freeze)
  with opengl: framerate is good
Any idea what causes it and how to fix it ?