This is probably due to poor 2D acceleration. Per default Ubuntu uses the framebuffer for console and this is badly emulated on VirtualBox/MAC. To speed-up the console disable/blacklist the VGA frame buffer in /etc/modprobe.d/blacklist-framebuffer.conf. To do this just add the following line: "blacklist vga16fb" and then reboot. Voila! the console should now be fine.