From your Documents/Paradox/Stellaris/logs/ folder, attach system.log .
There's some weirdness with your video setup here:
Current Mode: 1920 x 1080 (32 bit) (60Hz)
Monitor Model: SAMSUNG
Monitor Id: SAM0D3A
Native Mode: 3840 x 2160(p) (30.000Hz)
Whereas in the game you are using 3840 x 2160 at 60hz.
I suggest in the game you should use the same as your Windows desktop, 1920x1080 @ 60, OR the native mode 3840 x 2160 at 30hz.
Whether that has anything to do with your performance problem I'm not sure.
Please report the temperatures of your CPU and video (GPU)
1/ before starting the game up and
2/ after say 30 minutes' play.
If your PC's vendor has given you no way of reading temperatures, install
SPEEDFAN
http://www.almico.com/sfdownload.php
Run msconfig from the Windows taskbar search box. Click on Boot, Advanced. If the Number of Processors box is ticked, un-tick it.