I use Thunderstore mod manager, no issues. Same FPS as vanilla. I'll give the same advice I always give.. try completely removing drivers with something like DDU, and then reinstall. If you are still having issues, perhaps reinstall the game. If neither of those options work, it would seem you are having hardware issues.

Ryzen 2600 and GTX 970 SSC and depending on the area I can get over 100 FPS, most places average 70 FPS with only 55-60% GPU usage and highest thread usage is 60%.

If you have a decent multi-core cpu and ram, you can run multiple servers and the game on the same machine with no impact on performance.