Solution to delayed boot-up time on Linux

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A solution to a problem that have plagued Linux users for years:

As a user of Linux Mint Debian Edition since the release of LMDE 2, 8-9 years ago, I can reveal that never have any problems taken more time and effort to solve than this one.
If I remember correctly, this problem occurred for the first time with the release of LMDE 4, code-named Debbie. LMDE 4 Debbie was released on 20 March 2020.
In the early days when this problem first occurred, I spent considerable time and efforts to try solve the problem, but I couldn't find a solution, not before today.

If you search online on "slow boot-up time Linux" or similar searches, you will see that different tech forums are full of similar problems.
I have read many of those posts where users tried to solve the problem. But did any of them actually solve this problem? Nope..

Yesterday I started thinking that the problem must be related to the system's Display Manager, and it's complex system processes under the hood when your system is about to boot up.

And that proved to be the right idea, because today the problem have been solved. 

I had experienced that none of the "solutions" suggested in different tech forums online were able to solve the problem.
So I figured, I must try to install another Display Manager than the LightDM Display Manager that sits aboard the LMDE distro.

The Gnome Desktop Environment is used not only by Debian, it is also used on other distros, such as Fedora.
Since Gnome is the default Desktop Environment on Debian, I decided to pick GDM - The Gnome Display Manager - so I went ahead and installed this Display Manager.

Firstly let me upload a photo taken with my phone of my Monitor when the problem occurred before the problem was solved.
The problem becomes visible when hitting ESC, when you notice that your boot-up will be one of those slow ones, which always takes 1 minute and 30 seconds:



Now follows a more schematic procedure on what to do, if you see the above message with boot-up time delayed by 1 minute and 30 seconds:

Important:
The first thing you will do is to create a new snapshot with Timeshift.

Then install Gnome Display Manager with apt (in its current iteration gdm3).

Then issue:
sudo systemctl disable lightdm.service
sudo systemctl enable gdm.service
(the gdm.service may already be enabled if you chose to do so when you were prompted by Synaptic Package Manager to enable it when you installed gdm3 and dependencies packages)

Now reboot!

Boot up the system. Then reboot and boot up the system again a second time.

Now run the LMDE live system from a USB stick and restore the Timeshift snapshot you created just before installing GDM, in order to go back to your system where you had LightDM enabled as your Display Manager.

When you restore a Timeshift snapshot, your system should be restored back to exactly how your system was when you created the Timeshift snapshot.

So in theory, when following the procedure above, it should not solve the problem shown in the image above. But it did!
Somehow some hooks somewhere in the system must have been created and not reverted back when restoring the Timeshift snapshot.

When booting up the system again, I was instantly taken to the login window. 
I repeatedly shut down and rebooted the system 3 times. No more delayed boot-ups.
It was so quick that I could not read the new message that was printed on top of the Monitor.

I then recorded a video of the Monitor when the system booted up.
The new message that now appears on every boot/reboots is shown in the video snapshot below, with the system now booting up quickly, without delay:  





Problem solved!