First, I am so glad this is available on GOG. But I have given up trying to play it already. I really don't remember the mouse being this stutery when I was a kid. Is this really how it was back then or is it a problem with the GOG version?

I have heard the windowed mode might fix this but I'd prefer to play in full screen. :(

Was it really?

I get that the animations and everything are 20fps, but was the mouse really like that too?

The main menu of the game is still animated at 20fps, but the actual video output, including the mouse, is whatever you set it to in the config. So the mouse at the main menu moves at a nice and smooth 60fps, but then after loading into a game the mouse and video output get dropped down to 20fps. Is it not possible for the rest of the game to work the way the main menu does?

I really doubt it ran like this back in the day.....

It did.

Have you tried lowering your mouse sensitivity? I lowered it by about half for Diablo and it felt better.

same here

The entire game screen, including the mouse cursor, is only redrawn once every 50ms i.e. 20fps.
The menus aren't handled by the game engine so they operate at the regular frame rate.

DevilutionX now runs at monitor refresh rate which allows the mouse to move smoothly.