In WIP I can draw a line and Render speed is 3 fps. Then i open a scanned surface or other object and render speed i 30 fps. I can then draw same line again or import complex objects and speed is still fast.
I presume the scanned file opens with a rhino5 style environment. Those are much faster to render! (But does not have the smooth, soft skylight)
Have you tried resetting the render settings and/or importing the scan into a new document instead of opening it?
Yes, you are right Opening same line created and saved as R5, render ten times faster than if i draw it in R7. I got a core i7 professional, but it got only Intel HD graphic 520 card. I think i will need a new computer