While trying to set up Android x86 on VirtualBox on my Windows 7 laptop without VT-x as an alternative to the slow QEMU-based emulator and ARM image that ship with ADT, I noticed that Android was behaving extremely erratically in terms of speed.
Eventually, I figured out that it worked fine and booted in seconds if VirtualBox captured the mouse and the mouse was being moved. However, if I stopped moving the mouse pointer, the VM would practically freeze.
I did manage to dig up a bug report with the same symptoms, but it was eventually closed after the user who posted it failed to respond to request for additional information.
The VM is configured as Linux 2.6, 512MB RAM, 2GB HD file, and everything else as default.