Black screen at booting

What problem are you trying to solve by not showing anything on the screen? Surely you can't possibly think that the insanely tiny amount of CPU usage caused by playing an animation or printing text is slowing down your boot time significantly. It's problably something like a millisecond. It's an amount of time that doesn't contribute at all to the wait.

You'll probably get the biggest savings by using hibernate vs. shutdown.

Having said that there is software like bootchart in the repos to help analyze the boot sequence.

one thing i wanted in Trisquel 7 was to get a silent boot. if anybody is interested, my non-perfect workaround with plymouth is to change the colours to black (0x000000) in /lib/plymouth/themes -> text.plymouth and (im not sure if this was the reason in my case) to disable the access to the .png background picture by renaming, so that plymouth is only in 'text mode'. after that run 'sudo update-initramfs -u' to initialize the changes, btw: on shutdown this configuration has not the intended effect

second i wanted to make the boot faster. so far i figured out, to get a significant improvement, i would have to change the init to a faster one, thats not possible because of the dependencies

third just to mention it, my wish would be that trisquel would be based on devuan, not debian and at least not on ubuntu