[2020] Poor Performance

Has this always happened on this machine with this game? or only since when?

Does this also affect EU4 and HOI4?

DXDIAG is a program you run from a command prompt or the Windows start menu 'run' dialog box (or 'search programs' in Windows 7 or later). 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 your Documents/Paradox/Stellaris/settings.txt and pdx_settings.txt .
From your Documents/Paradox/Stellaris/logs/ folder, attach system.log .  

You could try changing settings.txt to say refreshRate=60 . That aside I can see no cause of this problem in those files.

Particularly odd that this does not affect similar games.

A weird one - what if you log OUT of the game launcher, or just run stellaris.exe by itself without launcher or Steam. Same issue?  

Please report the temperatures of your CPU and video (GPU)
1/ before starting the game up and
2/ in the main game menu
3/ after say 10 minutes' play.
If your PC's vendor has given you no way of reading temperatures, install
SPEEDFAN http://www.almico.com/sfdownload.php