I'm running Ubuntu 11.10 on VirtualBox and performance is really bad. It takes seconds to respond to a click. Is it possible to reduce the graphics rendering quality or other unnecessary things, to speeden up the gnome DE? (like in windows)
Yes, log out and select a different desktop. Unity 2D doesn't have compiz running so is faster in some circumstances.
Or install Gnome Shell (you only need the
gnome-panels package in 12.04) and you can select the Classic desktop which is probably even more efficient.
Or don't use Gnome at all. There are plenty of other desktop environments (LXDE, Xfce, etc) that are light.
If you haven't done it I would recommend you to install the Virtualbox Guest Additions.
These will also install a virtual graphics card that the guest Ubuntu will recognize and Jockey will automatically suggest a driver for it.
This will probably solve your performance problems due to graphics rendering.
To install it just launch your guest OS and from the menu of the VB window choose: Devices -> Install Guest Additions.
I also use Ubuntu on 11.10 on virtual box and there's some useful stuff in a guide that will help you, it's at: http://techod.com/how-to-make-ubuntu-faster/
All the best!
Here is a thorough general guide to speeding up Ubuntu in VirtualBox...
I was surprised to find that seemingly benign but suboptimal settings within VirtualBox itself can make the experience of Ubuntu in VirtualBox horrendously slow--even on high-grade equipment.