When you boot, there is a small count down before continuing with the boot process. This was originally introduced to ensure that older hardware load modules in time for the kernel to boot. Nowsaday, it is used for users to select their kernel modules, to dual boot etc.
However, if you have a modern machine and you wish to boot fast, you can edit
# /boot/grub/menu.lst
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timeout 0
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(Change timeout from 5 to 0)
You just save 5 seconds from your boot time
