If your Ubuntu uses Unity, then it is natural Ubuntu runs very slow in Virtualbox. Ubuntu and Unity is very heavy, especially on low RAM. So the solution is vary, one of them is changing your Unity into another lightweigth user interface such as XFCE, LXDE, or even Openbox.
How To Install XFCE
How can I install XFCE along side unity?
How To Install LXDE
How do I install LXDE / Lubuntu?
How To Install Openbox
The Google keyword for these is how to install ubuntu.