Today I ran a regular update of my Linux machine and after the next reboot I saw a continuous high CPU load caused by gnome-shell. Beside a couple of other updates, Entropy had updated Gnome and the Nvidia drivers. I checked the settings for OpenGL and found a strange setting:
$ sudo eselect opengl list
Available OpenGL implementations:
 xorg-x11 *
Fixing the setting by executing $ sudo eselect opengl set 1 and restarting the X server solved the issue: Since then the CPU load is back to normal.