I found the problem. Your system only has one GPU (GeForce 9600) which it needs the 'nouveau' driver to use. But you have disabled the 'nouveau' driver in your kernel command line:
nouveau.modeset=0
and so gnome-shell is using software rendering, hence high CPU. So this is not a bug.
Please remove that option from your kernel command line (/etc/default/grub) then run:
sudo update-grub
and reboot.
Separately, I suggest you will get much better performance using the proprietary Nvidia driver. It seems your GPU requires Nvidia driver version 340. To install that please open the 'Additional Drivers' app.
I found the problem. Your system only has one GPU (GeForce 9600) which it needs the 'nouveau' driver to use. But you have disabled the 'nouveau' driver in your kernel command line:
nouveau.modeset=0
and so gnome-shell is using software rendering, hence high CPU. So this is not a bug.
Please remove that option from your kernel command line (/etc/default/grub) then run:
sudo update-grub
and reboot.
Separately, I suggest you will get much better performance using the proprietary Nvidia driver. It seems your GPU requires Nvidia driver version 340. To install that please open the 'Additional Drivers' app.