When I first started playing the new 1.3 version of the game I noticed that my performance / FPS was a lot worse compared to the original release version of NMS. Toying around with the settings didn't really help much, so eventually, after trying a few other things, I downloaded a driver update for my GPU. Turns out that it made a big difference -- it's still not super smooth, but it's decently playable. I should add that I don't play big AAA releases often, so I rarely have a need to update my GPU drivers. I suspect it may be the same for many GOG users, so if you're having performance issues with NMS, and haven't updated your drivers in a while, give it a shot!

http://www.nvidia.com/Download/index.aspx?lang=en-us

http://support.amd.com/en-us/download