From that log file there's either a game bug, a corrupted save, or there are broken mods in use.
But I doubt any of those are related to your performance problem, which sounds exactly like overheating.
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 .
Please report the temperatures of your CPU and video (GPU)
1/ before starting the game up and
2/ after say 20 minutes' play.
If your PC's vendor has given you no way of reading temperatures, install
SPEEDFAN
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