VirtualBox, Speeding up a guest VM

virtualbox is a virtual machine technology that runs on many host operating systems

virtualbox offers a number of "device types" for storage controller

IDE slowest
SATA fast
SCSI faster
SAS fastest

choosing the fastest supported by an guest operating system can substantially speed up the vm

rules of thumb:
IDE for XP
buslogic SCSI for old Linux
lsilogic SCSI for 2003 or Vista and 2008
lsi-sas for Windows 7 and later

[ always make sure the  "Host I/O"  option is "checked" ]

existing virtual disks can be moved from one controller type to another

[ avoiding the infamous  STOP 0x7B  ]

for windows a new controller should be added to the vm while it is shutdown
boot from the original controller, let pnp install a driver for the new controller
shutdown and move the boot volume and all other volumes to the new controller

use IDE for CD/DVD mounting and to control the favored boot device order

for linux if after adding a new controller type
the boot kernel complains it cannot find any volumes

Reading all physical volumes
No volume groups found
Volume group "VolGroup00" not found
(Unable to access resume device (/dev/VolGroup00/LogVol01)
mount could not find filesystem '/dev/root'

a new controller should be added to the vm while it is shutdown
boot from the original controller and use  mkinitrd  to build a new initrd

See how-to-migrate-rhel5-kvm-to-hyper-v for a mkinitrd demonstration