Is it just me or does this game run incredibly poorly? I get severe fps drops anytime more than 50/60 units are on screen. Has it always been like that or is it the result of a newer system trying to run an almost 20 year old game? Any input on how to optimize performance would be very welcome.

Its GoG in general. Several of my games won't sync with Galaxy without constant logout/login. It's.... irritating.

Legacy DirectX broken more and more. General slowdan is an old Win8 issue. Actually sorta same for WinVista/7 for non d3d renders (ddraw or alternate like gdi).
Win8 broke everything influding d3d including d3d 9. In favor of DX10/11+ games few tweaks applied to better performance and compatibility with overlay apps.
And another tweaks applied for legacy d3d games supposed to make them work properly. But instead make them extremely laggy or non-noticable at first slowdawn. With input delays too. More you play more slower you notice. Or if you have Win7 you can find how faster game work there. Or if game allow that - switch to windowed mode and you see it going work faster.
Its important to mention that this delays and lags/slowdowns happen without FPS drops by FPS calculators.

DX9 and earlier games require 3rd party DX wrappers. GOG have own and differ builds of it bundled with some games updates. Did Mark of Chaos bundled with that?
If not yet, use Narzoul's 3.0.1 DX Compat wrapper, WineD3D, dege voodoo or many else to your chose. Then dont use GOG Galaxy to install and launch games. Galaxy not affect they playability and stability.

I probably too late, but it was working decently on 9600 GS in 2009, so it should not be a problem now.

Mark of Chaos is DX9 game, so my first attempt would be to use DXVK, it works well for me in many other games.