Insert the OS X installation DVD and plugin the USB Drive.
Open Disk Utility, under /Applications/Utilities.
Drag the “Mac OS X Install DVD” to the “Source” field
and the formated partition on your USB drive to “Destination”.
Do not drag the volumes or devices that state the size of the disk, it won’t work.
I’m not sure if this speeds up the installation but it sure should. Whether it does or does this process still helps.