Some interesting concepts, but some are basic
1) Try not to have any modules, built everything into the kernel, get rid of initrd
2) On Gentoo you can start daemons/scripts in parallel, run the scripts from shared memory OR have the option to use openrc, which uses C/C++ and binds a lot faster.
3) Don't start services not required.
Just with that in mind, I'm sure one can get VDR box with X to idle in 5 sec just with that in mind.
my 2c