EDIT: I have updated my guide with a more stable solution below. Try this first before you attempt the Vulkan guide.
https://www.gog.com/forum/slave_zero/improved_performance_for_slave_zero_with_dgvoodoo2

Hi everyone, I haven't posted on the forums in a long while. I see on a previous topic that people are reporting performance issues with Slave Zero, especially with FPS drops in the Sewer levels on the game. The Direct3D Patch and Peixoto's Patch didn't work for me.

I found a makeshift fix, and I say makeshift because I experienced a crash to desktop right after destroying the Nemesis cruiser while loading the next level. It was fine after restarting the game. So I can't guarantee it works 100%. If it causes issues just re-verify the game files, this should restore everything back to the original state.

All you have to do is download the latest nGlide (version 2.10 at time of posting) and install it.

https://www.zeus-software.com/downloads/nglide

Then you go to the following folder:

C:\Windows\SysWOW64

Under this folder use the file search bar and search for all of the "glide" DLL files and all of the "3dfx" DLL files and also the "nglide_config" executable. Copy these files into the Slave Zero folder where the old nGlide files will be.

In my case it would be here:

C:\Program Files (x86)\GOG Galaxy\Games\Slave Zero

Replace all of the files. When finished, double-click on the "nglide_config" executable, select Vulkan from video backend menu and save. Then start the game from your desktop, start menu, GOG launcher, whatever.

I attached photo of the file names below.