Hello Wiktor,
As I understood you're targeting to get the image of the special area of the simulatied section with color magnitudes in high quality.
After watching your video I suppose that a long time of creating color magnitude by your computer is due to a huge zoom.
For accelerate the process try to change zoom such that the section could fit into the screen completely.
You can increase the screen resolution up to 4K in the GPU or Windows settings before the running the drawing process of color magnitudes. Run a drawing process.Then do screenshot and crop that image as you would wish to get in Therm. It will allow to recive a quality image.
Regards,
Zugur