When I first installed K2 in a vm it ran ungodly slow. It of course was GNOME eating resources like a hog so I decided to use something a little lighter. After some messing around I settled on MATE.
apt-get install mate-desktop-environment-core
To set it as system default windows manager use this command
echo mate-session > /root/.xsessionNow just reboot and log back in. You should be dumped to the mate desktop and your Kali install should be running much smoother :)