I know it's been asked why Ubuntu is slow as a guest OS in VirtualBox, but all those answers are fairly old and seem to focus on graphics. In my case booting is also very slow, and there is a long delay when launching the software center (or anything) during which time the CPU runs at 100% but nothing is happening graphically. I've given the guest 2 GB of ram, 16 GB of disk space, enabled 3D acceleration (doesn't seem to make a different either way, though), gave it 1 CPU (I wasn't allowed to select more). Shutdown is also slow.