I dont have parallel boot enabled as well as it is disabled by default. Looking at this Systemd post triggered this thought to experiment with it one weekend. I have this laptop where I run Gentoo like a control freak. There is nothing in it other than the core system running as openrc services. So, I might play around adding readiness gates to those and see if anything at all can be parallelized.

> ... Linux boot time is so much shorter than whatever the BIOS does before it hands control over

On which system? Is your machine, for whatever reason, re-doing "memory training" everytime it boots? This is one can be really slow.