There's a few steps that will probably help you to fix it, most of the stuttering problems in this game is related to sounds decoding, so here is a little guide to help you solve this problem.

Unpacking Sounds.vfs from the game files maybe will fix stuttering for you

1. Download VFS File Extractor
You can find one here in this link or use one of your choice.
https://steamproxy.net/app/384110/discussions/0/135507548124664377/

2. Open your VFS File Extractor and follow this directory:(your steam folder)\steamapps\common\Pathologic Classic HD\data

3. Unpack Sounds.vfs and delete it afterwards (after unpacking it will show a folder called Sounds, showing all game sounds)

4. After the previous step, at the same directory open config.ini and change SoundsType = VFS to SoundsType = FS

5. Save everything and open your game.

6. Enjoy it.