Check unity support:
/usr/lib/nux/unity_support_test -p
In my case i got “NO” for “Not software rendered”.
Make sure you have installed following packages:
sudo apt-get install build-essential dkms
Check kernel:
uname -r ### to check if "generic" "server" or "pae"
Install additional packages depending on your kernel (in my case itwas already there for some reason)
apt-get install linux-headers-generic-pae
apt-get install linux-headers-generic
apt-get install linux-headers-server
Install Guest Additions (Devices → Insert Guest Additions):
sudo sh ./VBoxLinuxAdditions.run
Source: http://blog.jdpfu.com/2012/09/14/solution-for-slow-ubuntu-in-virtualbox