Maybe you should look at how init is performed by looking at 'SlackwareŽ rc init tour
'. It was produced with reference to 8.1 but the understanding of the 'rc' will aid to understand how the system is initialized.
The next thing I would suggest is to look at the kernel via the references in the 'Kernel
' section of 'Slackware-Links
But I really think to speed the boot time you would need to minimize your services first. Then look at how the kernel is handling your hardware. Some gain could be had (minimal) by setting up a initrd to handle the filesystem but your gain would not be much here.