I was seeing high CPU usage on a Whonix VM and the reason turned out to be enabling “Drag and Drop” by mistake. Even though this feature doesn’t seem to work with Whonix, dragging some desktop icons even just inside the VM while this feature is turned on causes continuous high CPU usage afterwards with /usr/bin/VBoxClient being the culprit
I doubt it makes any difference in a plain Debian VM? Looks like a VirtualBox bug to me.
You could try your luck on the VirtualBox bug tracker.