Bedges is correct. You can also use these speeds in conjunction with the setspeedmode command.
group setSpeedMode mode
Operand types:
group: Object or Group
mode: String
Type of returned value:
Nothing
Description:
Set group speed mode. Mode may be one of: "LIMITED" (half speed),"NORMAL" (full speed, maintain formation),"FULL" (do not wait for any other units in formation).
Example:
groupOne setSpeedMode "LIMITED"