I can confirm that I also get the result.
I run V7.10 Gutsy X86_64 on the host, on an AMD that supports hardware virtualization. My guest in this case is a Windows 2003 server.
I have tried the following: -
Enabling and Disabling Hardware Virtualization
Using the GUI virtualbox and VBoxVRDP
In all cases, the cpu load in the guest is 1-2%, whereas the host has a continuous 25-30%
Before I ran this under VirtualBox, I was running this exact same image with KVM (qemu) on the same machine and it ran with less than 1% CPU when the guest was idle. All I did was convert the qcow2 image to Raw, then convert it to VDI, then boot it.
Guest Additions have been installed with no difference.
Clipboard is disabled.
Something is not right.
Brilliant product apart from this however! Props to the developers.
If it helps (the developers that is!) I can make available both the virtual machine and the host (VRDP for the guest, NX for the host) for test purposes.