Why it takes so long to shut down?

I have a Skylake i5 6600K CPU, a Z170 MoBo and 8G DDR4 3000 .
Win 7 HP is on a Samsung 850 Pro and \Users is on a HDD (2xWD500 Block on RAID 0 array).
It's a powerful machine and everything runs without issues.

On a cold start, It takes about 40 Sec from POST to Skype log on.
What is strange, is that it takes 25 sec to shut down.

I can understand that to start it has to read and crunch a lot of data so I think that 40 sec is acceptable and I consider it's fast (Compared to the more than 3 minutes on a HDD).
Linux Lubuntu takes 9 seconds to do same thing. But it doesn't have antivirus, firewall etc.

On a Shut down it shows the "Windows it's shutting down" for about 22 seconds and apparently nothing is being done. Then you see the HDD LED flash for 3 seconds and it shuts down.
I guess that on a shut down it has to stop some services and then update the user register data on the disk.
What happens on those 22 seconds that apparently nothing is being done?
Again, Linux Lubuntu takes 2 seconds to do same thing.

Right click on the Task Bar & select Start Task Manager, select the Processes tab & see what is running just before you are going to shut down. Once you know what is causing the problem you can take measures to prevent it.

It a while since I have used Skype but I seem to remember that when you exited it was still running in the background & you actually had to manually close the program, but I can't remember the actual process.

Thank you Ranger.
There are so many processes and services running, that is difficult to know with one is the bad guy.
I stopped Skype and many processes and services and the results are the same.
Does anyone know if there is a way to make a shutdown log?

You could enable verbose messages to see if that helps.
Here's a MS article about this:
https://support.microsoft.com/en-us/kb/325376
I don't think this will give you the detail you want, but it's relatively easy to do.

If you really want to get geeky and get a shutdown trace log see these:
Trace Windows 7 boot/shutdown/hibernate/standby/resume issues - Windows 7 - MSFN
Gathering a Startup, Shutdown, Sleep, Hibernate, or Reboot Trace

Thank you David.
I will study and then work on it.
I'll let you know the results.

You're welcome !

Well, it took some time to come back.
I installed the Debugging Tools for Windows (x64) and traced the shutdown.

Does anyone has a suggestion on how to improve it.

On a Safe Mode it takes only 5 sec to shutdown.