I haven't tested any of these myself, but this page contains some great suggestions:

https://askubuntu.com/questions/10290/how-do-i-improve-boot-...

The initial boot delay is VERY annoying. I wish libreboot was supported on my laptop, then I could ditch lenovos ancient BIOS (and the NSA ping) once and for all:

https://libreboot.org/docs/hcl/#supported_laptops_x86intel