virtualbox.org

On host nothing is using CPU. If I close VB, usage drops to almost 0%.
Also host memory usage is low. Even with VB running, there is still some left free, as seen on the screenshot. (VB complains and stops the VM if all the required memory does not fit into physical RAM).

The guest is a Ubuntu Live environment. It is not like running a lot of services or so...

The CPU has 100 % because of trashing. Its a known problem for every environment with virtual memory and not enough physical memory.

Where I live "trashing" means excessive swapping/paging (exchange of data between RAM and hardddrive).
Do you mean something else? Because in my case there is no HD activity.