Make sure you have your BIOS

[blahblahblah]

Make sure you have your BIOS set correctly for virtualbox:

http://blog.darwin-it.nl/2012/04/bios-settings-for-virtualbox.html

Otherwise things slooooooww waaaaaay doooooown.

VMWare Fusion on a Mac OS X host performs much better than virtualbox in my experience. But yeah if you are building full Android that often you need every second, even a 10% hit will be too much.