Game FPS stuck at 30fps

Turn back the screen resolution & check in the GPU-Z program in which mode the PCI-E bus is running under load.

I'm not 100% which indicator you are asking about, I think it's PCIe x16 3.0 @ x16 3.0.

Full screenshot attached

During the game, turn on the AMD Adrenaline overlay (Alt + R) and see how many percent of the GPU is loaded.

I have Adrenaline overlay disabled because it's notorious for reducing performance, but this is the info I can gather.

Your RX 580 is physically hitting its limit at your current settings.
Shadow Quality: Set to Low or Medium. This is the biggest performance killer for the RX 580.
Anti-Aliasing: Switch to FXAA or Off. (SMAA or MSAA are too heavy for this card in 2026).
Ambient Occlusion: Turn this Off. It adds nice contact shadows but eats a lot of GPU power.
Press Alt + R -> Gaming -> Graphics.
Enable Radeon Image Sharpening and set it to 80%. This makes the game look "High Res" even if you lower some settings.

I've made the changes to the settings to little effect.

I do know that my 580 is not hitting its physical limits for sure. It's only drawing 90W out of its 185 TDP and the fans are practically on a vacation in Hawaii. I've seen this GPU pushed harder in benchmarks and other games, where its draws more power and the fans spin much faster. So it doesn't make sense how it is hitting its limits in COI and maxes out at 90W TDP when it can draw 185W in other games.

Is Fullscreen mode selected in the game settings? Turn off AMD Radeon Enhanced Sync. Turn off Radeon Chill.

I've tried both fullscreen and borderless, no difference. I have practically no settings on in Adrenaline. No Enhanced Sync, no Chill, no anti-lag. Settings attached.

Try this
In Windows - Settings > Gaming > Game Mode and turn it off
Set Shadow Quality to off and Terrain Detail to low, does the FPS stay at 30, or does it move up?

Done both, the FPS is unchanged. If I'm all the way zoomed in, fps goes up to about 40, which suggests that it's not a cap problem. It seems to me that something about COI, and only COI, is not letting my RX 580 use its full power.

In Windows go to Settings > System > Display > Graphics.
Click "Change default graphics settings".
Turn OFF "Optimizations for windowed games."
Turn OFF "Variable refresh rate" (if visible).
Open Device Manager (Win + X)
Go to System Devices.
Find High precision event timer.
Right-click it and select Disable device.
Restart your PC.

While the game is running, press Ctrl + Shift + Esc
Go to the Details tab.
Find CaptainOfIndustry.exe
Right-click it and ensure "Efficiency Mode" is unchecked
Also, Set Priority -> High

Made all these adjustments, no difference.

I've also reached out to AMD support directly and provided the link to this thread for further troubleshooting.

Check in any of these games, they give a similar load to the graphics card:

• Workers & Resources: Soviet Republic
• Dyson Sphere Program
• VoxelTycoon
• InfraSpace

As I said before, Dyson Sphere Program runs very well. New games run at 60 fps, I've gotten to end game still maintaining 60 fps. I do not own the other games so I cannot say. But I can say that more demanding games run better than Captain of Industry. I can run Enshrouded at 50 fps. I can run Stellar Blade at 60 fps. Satisfactory at 60 fps. Factorio at 60 fps.