My CPU is i7 dual core. I am running Ubuntu on VirtualBox. İt is extremely slow than other Linux versions such as Edubuntu 13.04 and its previous versions before 12.10. The 12.10 was also very slow.
It's because if Ubuntu can't use graphics card for acceleration (probably that's what happens in Virtualbox), it uses CPU for rendering graphics trough LLVMpipe. It makes it appear really slow.
In Ubuntu 12.04 and earlier Unity2D was used if hardware graphics acceleration couldn't be used, but in 12.10 and 13.04 Unity2D is dropped in favor of LLVMpipe.
Some desktop environments that don't need compositing (like XFCE) are still fast even without graphics acceleration. That's why few Ubuntu derivatives, like Edubuntu, Xubuntu, Lubuntu, etc. are still fast in Virtualbox.
But if you install Virtualbox Additions, 3D acceleration will be available to Ubuntu inside a virtual machine and it will run at normal speed.