I believe in the original VirtualBox gui its located under "Runtime" TAB and shows various information about guest runtime.
Attached is the screenshot showing this tab I found after some googling.
I've added some code to my phpVirtualBox (v4.2-5) so it shows the guest performance. if you want I can add the changed files
in the attachment is a screen shot of how it looks (single selection)
it can show all the data for multiple vboxes and auto updates it
Added patch files for phpVirtualBox v4.2-5 (there are comments for adding more metrics)
I would love this. Showing the CPU load for the host machine would be a great addition as well!
Also a patch package for version 4.3-1
Is there a simple way to incorporate the 4.3 patches? This is a great addition.
Would be great to have performance stats added to the Host also, beyond the current RAM usage.