about 2 months ago - AndrewT - Direct link
FYI this game is NOT restricted to a single core.

FPS per se is not a good measure of game performance for a no n-action game like this, but if you get stuttering and slow menu responses that's a different matter of course.

Right click on stellaris.exe , properties, compatibility. Tick "disable fullScreen optimisations" untick every other box in that dialog, Apply and exit.
Then in the launcher Game Settings menu, select fullScreen display mode, Vsync off, refresh rate 60.

Is that any better? If not,
DXDIAG is a program you run from the Windows search box on the task bar. After running it will open a window and start collecting info with a progress bar in the lower-left corner. When it completes click the 'save all information' button and save it to a file then attach that file here.

Please attach here your Documents/Paradox Interactive/Stellaris/settings.txt and pdx_settings.txt .
From your Documents/Paradox Interactive/Stellaris/logs/ folder, attach here system.log , error.log .  
about 2 months ago - AndrewT - Direct link
The refresh rate is set to 144 there, can you make it 60 please?

And you may as well update that AMD video driver, i think there's a new one out this month
about 2 months ago - AndrewT - Direct link
I don't know offhand what else could cause this to you , I am sorry.

In Windows run msconfig , boot, advanced. If Number of Processors is ticked there, untick it and reboot.

Assuming that doesn't help, what if you exit the Steam app entirely then run stellaris.exe directly from File Explorer? Is that any better?

And we had one case recently where running Task Manager helped performance, weirdly!