In 2008 MontaVista presented embedded Linux that boots in 1 second. In reality it was striped out version of Linux that won't work on Desktop or Server computers.
Linux can boot faster if specific configuration file (or inside core.gz) will be loaded by bootloader. Instead probing hardware everytime computer boots, configuration file will provide informations about hardware. This solutions require cooperation of Linux kernel developers.
Distros developers could also create this kind of configuration file that contains informations about hard disks, cdroms, floppies, static ip, resolution,, filesystems and services. TinyX and TinyCore is example how optimalisations improve boot time, but boot time can be even faster than is nowadys.
http://embetrix.com/2017/05/16/embedded-linux-fast-boot-techniques/