Ok, after reading your replies I made some more tests and I found an interesting thing:

As long as no other window is overlapping PS window, it updates smoothly in realtime as when I play it.
This even if I type something in the other wndow or I run something in it (it was a terminal).
As soon as I move the terminal window (or any other one, even a popup from the window-manager menu) in front of PS window, the latter freezes.
Even if I just overlap one pixel . It loos like as soon as it detects that some of its surface doesn't need to be updated anymore, it stops altogether...
And then it takes some 2 or 3 seconds to recover. The same happens if I switch to a different workspace.

Note that the same behaviour DOESN'T happen in the character chooser phase: my elf turns around quite happily even if I partially cover the PS window.
This last thing makes me think that the behaviour depends on PS application, and not on the window manager or other environmental variables as somebody suggested...

Just a note on my configuration:
Fedora Core 6
Gnome
Self compiled PS (on a FC5 installation)
The PC specs are not so bad:
Asus v6j, dual core T2400 2Gb RAM, NVidia GeForce Go7400

So I don't think the behaviour is due to low specs...