Snap can slow boot time down. If you are not using any snap packages, you can remove it. (Removing Snap is the first thing I do)
Check if using any snaps:
snap list
If you do not recognize anything as something you installed then it is safe to remove:
sudo apt remove --purge snapd
You might also:
systemctl disable NetworkManager-wait-online.service
That service is for Ubuntu Server. It's probably not something you need unless you are running a Server...
Shut Down of Computer:
sudo nano /etc/systemd/system.conf
Change #DefaultTimeoutStopSec=90s to DefaultTimeoutStopSec=4s being sure to remove the hashtag.
Ctrl+x to exit, then y to say yes to save then enter key to save.