1.) I do not use any overclocking; in fact I have never heard of it, before you said it
This is goody.
2.) Local disc (C: ) - ProgramFiles (x86) - GOG Galaxy - Games
Possibly a factor. Move the game's installation to the root of your drive or a folder of your own creation. For example:
...C:\The Witcher 3\... or
...C:\MyRPGgames\The Witcher 3\... (Installing games to
Program Files or
Program Files(x86) can be problematic as these are protected directories. Not always friendly for gaming.)
Also, ensure that
witcher3.exe is set to "Run as administrator". Set the Permissions for
...\Documents\The Witcher 3\... to "Everyone".
3.) Avast ( but just the free version9
This is goody. Just be sure the game's directory and the
...Documents\The Witcher 3\... folder are added to the exceptions list in
Avast!.
4.) If you ask what kind of I'm using: a Samsung UE32EH5000 (tv)
Might be a factor. Ensure that the TV is set to "Game Mode" if it's available. (This will be in the Options Menu on the TV itself.) Also, if the TV has any built-in "anti-hacker" features, try disabling them to see if that has any effect.
5.) If I recall right I didn't dowload anything when that happened
Well, anything is possible, but this is a very old issue, as I mentioned before. For it to have surfaced now...something with your system is not getting along with the game. Windows can often sneak updates in when you're not watching. Normally, I disable auto-update, but Windows 10 is still going through its growing pains, so I think it's best to just leave it for now.
Most surprising to me is that you see the slowdown if you begin a new game. That definitely should not be happening. Be sure you send a ticket into CDPR Support if you haven't already.