very strange X11 behaviour during upgrade

[Pierre Frenkiel]

I had a very strange X11 problem while running update-manager to upgrade
from gutsy to hardy.

the 1st step (downloading packages) was normal,(it took about 40 minutes
for 1.6 Gb). Then, it started installing the packages, with an
estimated remaining time of 2 hours, but after about 15 minutes, it became
almost frozen:
   - the setting for every package took 2 to 5 minutes.
   - in an other xterm window, when I hit <CR>, the line feed came 30
     seconds later, but if I moved the mouse(without any click), the line
     feed came immediatly !!!
   - it's not a problem of machine load, because if I made an ssh connection
     from an other machine, the response was immediate, and anyway, top
     showed a load < 0.1

  this lasted 12 hours (twelve !), and about 15 minutes before the end, the
  speed became suddenly perfectly normal.

  Has anybody a possible explanation?

Pierre