Just upgrade from Ubuntu 17.04 to 17.10 in VirtualBox 5.1.26, very slow. It is unusable. It is running 4G RAM for Ubuntu 17.10. Any idea?
I had the same issue (8GB ram, 4 cpu's). The
gnome-shell process constantly uses 10% cpu.
Disabling 3d acceleration did fix it (I didn't have 2D enabled as Virtualbox complains about it). Before you disable 3d acceleration be sure to disable the Virtualbox graphics driver under "Additional Hardware" inside of Unity. If you don't do that gdm3 won't even load.
The Unity desktop is still available and works fine, but one of the reasons to upgrade is to use Gnome. Even logging into unity and doing Additional drivers didn't help gnome.
It seems that the Wayland display server may be the source of this bug.
I already had 2D and 3D acceleration deselected and was still getting very slow performance from Ubuntu17.10.
Clicking the gear icon beside the "Sign In" button on the log in screen gives the choice of using the default display server (Wayland) or Xorg and when I chose Xorg the flickering and high cpu usage all disappeared without having to switch to Unity.
Apparently the forthcoming 18.04 LTS will revert to using Xorg by default