«
I think it proves that with the same hardware and libraries (almost,
xorg, cairo, pango etc are the same, but gnome-panel uses gtk3 while
mint-panel gtk2, afaik),»
No, it really doesn't prove anything at all. In Ubuntu 10.04, we used the Compiz 0.8-series, I believe. That was certainly _much_ faster on some systems than 0.9 has been, at least until recently. Things like that can play a big role. Have you tried using Metacity in 12.04 to see if that makes a difference? Since you're comparing different systems at different times, there can also be completely unrelated things going on in the background.
