Fast startup makes hibernate and turning on your computer faster than reboot.
You don't need fast start-up (the windows version, not bios) if you have an nvme or ssd. They are so fast it is about same speed to start from powered off to windows desktop. On my PC anyway.
I don't think it has any direct effect on hibernate speed. They do both use hiberfil.sys to store the files it uses. They are different functions.
Fast Start-up only saves the kernel and drivers, discarding user apps to start fresh, while Hibernate saves all open apps and files, resuming exactly where you left off.
so depending what you had open before you hibernate, it can take a little longer, whereas fast start-up is just like a normal start-up.
I normally only have the same programs that I have set to auto load with windows now, that I doubt I notice a difference at start-up between fresh and hibernate.
Its shut-down where hibernate skips all those things you mention where I am seeing the change.
I remember, if you only used Fast Start-up, you could run a command to shrink hiberfil.sys to save space. Not really important to this discussion.
MS uses the Store to update applications as well, they happen in back ground. Uses same service to update games.
Windows does things on reboot like cleaning the component store, deleting old update files, reconfiguring drivers, etc. Some of these things take awhile
It shouldn't run them every time you turn pc off, only maybe once a week or so. Its more likely to do all that during the restart after an update.
Seems MS just added another bug when they fixed the last lot. The patch last month fixed a stack of bugs. Whack a mole is a MS bug hunters job.