The drives that have the setting for a SINGLE drive basically close the loop on whichever wires are used to detect the drive's setting.
This will speed up boot times. Also, if you have a drive set to SINGLE and have another HDD ont he same cable behind it, it won't be detected.
Been burned by this before.
Another way to speed up boot times is to set your IDE channels that have no drives on them to NONE in your BIOS setup. If the BIOS is set to NONE it's not even going to look for a drive on that channel.
Turning off FDD seek & FDD swap and enabling the QUICK POST options will also speed up boot times as well.