Running Ubuntu in a VM, I've been unable to update to 13.04 due to them dropping Unity2D. My VM doesn't have enough video memory to run Unity 3D, and that's not going to be changing anytime soon, so I'm stuck on an old version.
If you're not too attached to Unity, try Lubuntu or Xubuntu. Both work rather well in a VirtualBox. The last Ubuntu that worked lightning fast in a VM was 11.04.
I moved on to Crunchbang. It's noticeably faster than even Lubuntu in a VM. I don't think even the older Ubuntu's were this fast. I dedicate 32M of video to Crunchbang and have had no problems.