Speeding up the whole system and optimizing GstarCAD are the better ways to make it work lighter and faster.
FILLMODE is very useful for drawings with hatches and solid fills. If you feel hatches degrade your computer performance, you can change FILLMODE to 0. This variable is also stored in drawing.
If you embed or reference images in your drawing, you can feel that panning and zooming are slow. Especially if the images are high resolutions and you have many of them in drawing.
You can change RTDISPLAY to 0. By default, the value is 0. But probably you or your coworker turned it on before to see the image when navigating drawing. This value is also stored in registry.
This is for you who need to work in layout with many viewports. You can set maximum visible viewport contents. So if you have issue with performance, you can set it to low number. It will not affect your plot result.