PROBLEM: SSH is disconnecting after a short time!
SOLUTION: Edit SSH server configuration and increase timeout.

[?] Boot what?
https://www.computerhope.com/jargon/b/boot.htm

It is good to reboot the machine and run these 3 commands to understand what is happening during boot and how fast it was.
This will show kernel and userspace boot start time.

root@server:~# systemd-analyze

OUTPUT:

Startup finished in 3.972s (kernel) + 9.259s (userspace) = 13.232s
graphical.target reached after 7.717s in userspace

Show services startup order:

root@server:~# systemd-analyze blame

And to show very detailed report run:

root@server:~# dmesg

And now you should determine what is slowing down your boot!