My problem is as it says in the title, I'm getting rapid drops in frame rate all the time, without any input. The game opens in fullscreen, so I thought I might try windowed mode, but I can't even find the .ini file to make any changes (I read somewhere that there should be a sshock.ini file, but this does not exist on my computer), so I guess my issue is twofold. I installed SS1 through GOG galaxy, not sure if that's supposed to make a difference.

not seeing the ini files would suggest the classic edition.

Oh dang, I had both installed and somehow didn't notice I was opening the Classic edition :/

Sorry all, false alarm, crisis averted, all clear, move along. Many thanks for all your help and support during this troubled time.

Incidentally, just out of curiosity, although not within the scope of the original question, does anyone know why I might be getting this performance in the classic edition (whilst the REAL enhanced version works fine) ? It might help for future troubleshooting.

The Classic Version runs on the DOSBox Emulator. You might have to lower or raise the cycles(CTRL+F11 or CTRL+F12) in order to get the best performance for your system.

You can read more here:
https://www.systemshock.org/index.php?topic=2213.0

Have fun Hacker :)

Ok, I might have a play around with that in the classic version as well. Many thanks once again!