If you are using Ubuntu 14.04, and fed up with the slow speed of compiz (Ubuntu xwindow manager) especially in the virtualized environment, there's a solution. Just boot into the text mode.
First step is to edit /etc/default/grub as follows: (sudo vi /etc/default/grub)
After that, do sudo update-grub. Once finished, just reboot. That's it. Now you can boot directly into the simple 'text' mode.
If you want to start XWindow manager after boot, all you need to do is:
If you want to turn that GUI down, you just need to stop the service.
Then, you might see some cursor flickering with no respose: don't panic, just press Ctrl-Alt-F1. Then you will be returned to the original screen that you started the lightdm service.