I was experiencing a fair amount of lag during my interaction with the KDE desktop. To fix this I unmasked the latest hard masked nVidia drivers available in portage ( wich are also the latest upstream drivers provided by nVidia. )

echo “x11-drivers/nvidia-drivers ~amd64″ >> /etc/portage/package.keywords
echo “=x11-drivers/nvidia-drivers-260.19.12″ >> /etc/portage/package.unmask

After emerging them, reloading the module and starting X again. The lag was gone. Now I have fast and pleasant to work with KDE desktop. Also my FPS is up to 10344.223.