Classic Shell doit se configurer pour le nouveau système.

And for the past few weeks, my computer has been very slow, I haven't changed anything.

It could also be an infection on your machine.

--
"We gulp down the flattering lie in great gulps and we drink the bitter truth drop by drop."

Yes, I have this photo at the bottom left of my screen.

Hello,

Classic Shell becomes Classic Start which in turn became Open Shell.

Why switch from Classic Shell to Open Shell?
The developer of Classic Shell has stopped developing this software.
Classic Shell will no longer be updated and will not work with future versions of Windows; it may no longer function due to updates and bugs that could crash your system.

That's why you have this message:
Classic shell needs to configure itself for the new system

1- Save the configuration of Classic Shell.
Optional, you can skip this step.
Right-click on the Start button -► Settings -► Backup.
Save in an XML file.
Go to the installation folder of Classic Shell -► C:\Program Files\Classic Shell
Back up the following files and folders:
The folder: My_Icons
The folder: Skins
All button files: .png

2- Uninstall Classic Shell.
It may be necessary to kill the process; right-click on the taskbar, look for classic shell, right-click and end task.
With if you wish --► Revo
it automatically kills the process, then proceeds to uninstallation, do the advanced scan and check all boxes to remove registry keys.

3- Install ==► OpenShell (version 4.4.152)

4- Set the parameters menu to French --► https://www.cjoint.com/c/JJdpbm0YlyW
Unzip the zip file.
Place the file fr-FR.dll in the installation folder of Open Shell: C:\Program Files\Open Shell
Right-click on the Start button / Settings.
Tab ► Language
Select the language ► fr.FR-French (France)
Click on ►Check for updates
Then on ► OK

Source.
I think this is quite complicated for a beginner.



5- The best option is to stick with the Windows 10 Start button.
Create a restore point if you need to uninstall Classic Shell.

For the slow performance issue, before going to the virus/security forum, make sure Windows Update is up to date, meaning today it is version 2004.
Go to Settings → System ► System Information
And use the antivirus already integrated into W10, which is Windows Defender.

A+