I've come back to the game after several months and now I'm experiencing stuttering bad enough to make the game unplayable while using my 360 controller. The game literally seems to pause for a few frames whenever I press a button, move a stick, or press a trigger. The stuttering is entirely absent and the game runs perfectly while playing with the mouse and keyboard. Curiously, the game still runs fine while using a Steam controller, even when it's set to use xinput controls. It's only the 360 pad that's affected. Help? :(

Windows 10, nvidia GTX 780 (latest drivers), wireless 360 controller, Witcher 3 from GOG 1.12.1

Big thanks, works as a charm :)
At first anyway, then it started to stutter again but not as much as before. I will go with mouse and keyboard for a while and see if i encounter any stuttering.

Yep, you're right. I tested quickly and it seemed to help, but didn't really solve the problem. I found the real solution just today:

(0. Make sure the Windows account you're using has Administrator privileges)
1. Press Win-X, Open your Device Manager
2. Find your xbox 360 controller near the bottom
3. Right click, Update Driver Software
4. "Browse my computer for driver software"
5. "Let me pick from a list of device drivers on my computer"
6. Pick the driver with version 2.1, not 10.
7. Restart computer and you should be good to go.

Thanks, that worked just fine :)