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.

I have to wonder, what is Thunderstore mod manager doing that is so CPU/video card intensive that it brings my FPS to such a low level? I thought all a mod manager does is install mods, and then make sure they don't conflict with each other.

I did some testing with thunderstore mod manager, both with mods and without. There was not a big difference between them. range from 8.2 to 12.9 FPS after about 2 min of testing.

Then I tried it with steam dx, and it was a huge improvement 10.8 to 17.4 FPS
Finally I tried it with steam Vulkan and it improved again 13.2 to 18.9 FPS

It seems thunderstore mod manager is bringing down my FPS. I had been using vortex with other games but switched to thunderstore when one of the earlier mods I wanted with Valheim suggested I switch. Now it seems I need to go back to vortex, or another mod manager, or install the mods myself.

For the people that use mods, which mod manager do you use, or do you manually install them yourself?

How does everyone launch the game, from steam, your mod manager, or desktop shortcut?

My current video settings:
Resolution: 1920 by 990 60 HZ
vegetation quality: low
Particle lights: high
Draw distance: med
Shadow Quality: low
Bloom: on
SSAO: on
Sun Shafts: on
Motion Blur: off
Tessellation: on
Soft Particles: on
DOF: off
Anti-aliasing: on
Chromatic Aberration: on
Vsync: off

The only difference between our settings is my bloom is off.<draw distance is maxed on mine>
https://steamcommunity.com/sharedfiles/filedetails/?id=2499319810
If I can keep a solid 60 consistently then I guess it must be the mods. I run totally vanilla.

The only other thing I can think of, if you must run the mods, is to play at a lower resolution.

I had a NVIDIA driver update do this to me a few months back. I am on a 3080 and updating the driver took me from 60-80fps < 15-18 fps. Try upgrading, or in my case downgrading the drivers. This was the only game affected and the next update was OK

If you are playing with mods and using thunderstore mod manager I don't think I have a choice? I have seen the choice when I launch with steam. Is their another mod manager that allows you to chose between vulcan and dx versions?

I am now thinking some of my mods might be slowing down my system. Are their any mods that are particularly bad in slowing down the FPS?

I think there is also a way to force vulkan , a combination of editing the config file and the launch arguments.

Not unless you're trying to run the multiple worlds simultaneously.

The question seems to imply that saving multiple worlds with different seeds would have less impact somehow?

Have the same video and ram but have an I5

To keep a solid 60fps I run at 1920x1080 capped to 60 Foliage and shadows on lowest settings motion blur and depth of field off. Max view distance.

Running with max shadows and foliage made it a bit choppy at times so I turned it down for better performance.

I have a bunch of world seeds saved but each one seems to be its own rendering, having multiple of the same seed should not impact anything at all.

Are you trying to host a server and play on it? Never tried that myself.

What settings do you use for graphics? Resolution? Capped frames?

No, it does not.

the thing that mainly lower atleast my fps is buildings and terrain modifications.. so if u build on one map and do all farming and killing on others ur fps should be good untill u go home to ur main map with your massive castle. also try the switch between vulcan version and dx version.