Oracle is known to be resource hungry. I've heard of such issues but those were in cases where the Hardware was just meeting the minimum requirements. I am not sure of your hardware setup.
But you have indicated that you are running it on Virtual Machine? That could actually slow down Oracle, if i'm not wrong. VM / OS is not among my areas of knowledge But i guess a major portion of the Process time / Memory is shared between the underlying OS and the OS running on VM. Finally Oracle might be left with little. Hence the slowdown?
In addition to following schwertner suggestions, if you could also try to setup on a standalone OS and see if there are any performance variations, it can throw some light.
