gnome-shell faster in ubuntu gnome than in debian

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http://askubuntu.com – i'm running, for trying, both debian and ubuntu 12.10 gnome remix, on virtualbox. Both the virtual machines are set up in the same way. The strange thing is that gnome shell is faster, for what vbox can do, in ubuntu gnome remix than in debian. Also, in debian always start fallback mode and I have to press alt+f2 and type [code]gnome-shell --replace[/code]. In debian is incredibly slow, full of lag, in ubuntu gnome remix quite better and faster. I'd like to understand why (HowTos)