Here are some more pieces of information.

* indeed there is no corruption, and it is quite stable (at least as
stable as in the single screen config)

* if the "rotate" option is in xorg.conf, or if I do it manually after X
is launched, with xrandr, the result is the same.

* if I disable DRI in xorg.conf by adding
Option          "DRI" "false"
then X won't start (black screen).
However if I start without DRI AND without rotate, then I can rotate with 
xrandr, but the result is the same as before (slow).

* the option 
Option          "NoAccel" "true"
has the same effect and disabling DRI above.

* the option "AccelMethod" "EXA" or "XXA" has no visible influence

* I have also tried to disable the laptop screen by xrandr --output=LVDS
--off, but the result is still the same.

I'm really out of clue here, sorry. I hope someone can make sense out of
it.


** Attachment added: "xorg.conf"
   http://launchpadlibrarian.net/11327541/xorg.conf

-- 
xorg very slow when rotated on external monitor
https://bugs.launchpad.net/bugs/174650
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