Why are you disabling this? If the desktop environment (DE) you're using
(which is it?) supports compositing, that might alleviate the problem.

Disabling AIGLX isn't a good idea either. I don't think Option "DRI2"
has any effect.
[...]

Does commenting out these options help?

I recommend against disabling this.

This Virtual directive is unnecessary with KMS and causes the driver to
allocate at least twice as much VRAM as necessary for the front buffer.
Please try commenting it out.

The Xorg radeon driver doesn't support UMS anymore.

Hi, thanks for respond so quickly.

In resume: The only thing give me an usable X environment is the following
option:

Option "Accel" "off"

This way I can work decently but not faster than with previous version
(Debian 7)

A detail that I forget to include is that I work with an external monitor
controlled by xrandr. When I start the X window system (writting "startx")
both screens are twins (it shows the same with the same resolution).  The
problem comes when I turn off LVDS and get the VGA maximum resolution:

xrandr --output VGA-0 --auto --mode 1920x1080

NOTE: I insists that with the previous version of driver (Debian 7) I didn't
have any problem, all works fine. All stop working with the upgrade.

If you need additional information, please feel free to ask

Thank you for you time.

I respond on your message below...

I really don't know why I disabled this, I just was proving different
options.

I'm working with the OpenBox window manager.

No, the only thing that help something is turned off accel:

Option "Accel" "off"

If I do that, I can get an usable X environment. Which it is paradoxical, because I'm working without graphic acceleration

I follow your recommendation, but with no perceptible effects

----- End forwarded message -----

There's no going back to UMS, you have to find some kind of solution
with KMS.

Can you try if a simple compositing manager (e.g. xcompmgr -a) helps
with the problem (with the options below commented out)?

Please provide /var/log/Xorg.0.log corresponding to commenting out
everything I recommended in xorg.conf.

Your GPU has a very limited amount of VRAM, so the kernel radeon driver
probably has to migrate buffers between VRAM and GTT a lot, which slows
things down. Disabling Option "EXAPixmaps" (which is the default for
that small amount of VRAM) should avoid that.

OK let's try it work... :)

I probed this program and really improved the window manager performance
with the Xorg accel option and with the options you say commented out

In return performance like mpv is now worse (the movement blinks and still
shots are slow)

I attached the Xorg log file with the suggested configuration and running
xcompmgr -c

Note that I suggested xcompmgr -a, which should be lighter than -c.
[...]
[...]
[...]
[...]

These are all defaults, no need to specify them explicitly in xorg.conf.

I missed this before. Why did you disable this? Does not disabling it help?
[...]

This is probably why mpv isn't working well. Any chance you can try if
the attached xf86-video-ati patch helps for this?

Note that you can always use mpv -vo x11 to get basically the same video
experience as with Option "NoAccel".

performance between "-c" or "-a" opctions.

Now I'm proving with: "compton -CG --config /dev/null -b &"

And I encounter relative better performance too with <Accel option "on"> in
xorg.conf file.

I don't understand why I need to use a window composition manager to get
better performance, but I suppose is another history...

I deleted this option. Now I have only this options on the "Device" section:

Identifier      "ATI Technologies Inc Radeon Mobility M6 LY"
VendorName      "IBM ThinkPad"
BoardName       "ATI Radeon Mobility M6 LY"
Driver          "radeon"
BusID           "PCI:1:0:0"

Option          "Accel"                 "on"
Option          "ZaphodHeads"           "LVDS,VGA-0"
Option          "AccelMethod"           "EXA"

# Otras opciones
Option          "monitor-VGA-0"         "VGA-0"
Option          "monitor-LVDS"          "LVCD"

With "-vo x11" option does not improve performance. I will try with the
attached patch.

Thank you