Unity vs KDE in Virtualbox

The other day I installed the latest Ubuntu 13.04 under a VirtualBox virtual machine using Windows as host. To my surprise, unity failed to launch properly on the virtual machine reboot, with compiz complaining, something I have sometimes seen on my work laptop. It's more surprising in a VM since it is in a way much more standard (no strange graphic card, no strange driver, the same stuff for every VirtualBox user (maybe I'm wrong there?)). I therefore installed KDE as a way to bypass this issue. Not only it worked, but the UI was much faster: there was some very noticeable lag in Unity, slow fade in fade out effects, when it worked before the reboot.

I am no hater of Unity, it looks well polished, nice to the eye and I use it on a home computer. I find KDE looks a tiny bit less nice, although I prefer the standard scrollbars of KDE. I wonder if others have the same dreadful experience with Unity under VirtualBox.