I have just installed an 12.10 64bit installation.

Same picture, running kdbg via ssh x forward is significantly slower
than with a 10.04 32 or 64 bit installation. I tested agains a (new) 32
bit installation of 10.04 on a Pentium 4 old PC, which still performs
significantly better than 12.04 and 12.10

I came across 2 things here:

1- it might be pure network performance plus the fact that simple X
applications like ddd probably (i don't know) use less network
ressources than QT applications do (more effects)?.

2- Is there any debugging you can propose, or maybe X benchmark / QT
benchmark, that could help to find out where the time gets lost

3- visually, the slow performance shows in some kind of 'rolling down'
effect of the application when starting it up (kdbg is our choice for
this test). when using 10.04 the window just pops up. With 12.04 and
12.10 you first see the top line, then the rest of the window is rolling
down, just slow enough that our eye can see it rolling. Still, using the
sliders and menues gives that feeling of a slow machine.

4- we didn't change Network or hardware between 10.04 and 12.04 upgrade
on the 64 bit machine in question. We even tried with a directly
connected remote X client via ssh, physically only one net cable between
them, no other network connections established. There must be a problem
somewhere ...

5- other people have seen similar, but maybe never experienced enough
trouble:
https://answers.launchpad.net/ubuntu/+source/xorg/+question/213691

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https://bugs.launchpad.net/bugs/1077965

Title:
  qt4 applications over X forward run slower than with Ubuntu 10.04 LTS

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