Have you confirmed that it actually uses (plain) GDI and not OpenGL? If it does use OpenGL, the problem could simply be that your driver no longer supports OpenGL, or has worse support than the driver you used on XP. Try running the OpenGL extension viewer and see what options you have in the dropdown box at the top.
Another thing you can try is using Dependency Walker to see if OpenGL32.dll is imported by the application. But this can be a rough edged sword since the DLL may be imported by a library DLL, or conditionally in which case it won't show up.
If you find out that the program is using OpenGL, and that your driver lacks proper OpenGl support, you can try scouting for a different driver that does. If no better driver is offered, you may consider trying TitaniumGL, which emulates OpenGL through Direct3D.
The relevant links:
http://www.realtech-vr.com/glview/
http://www.dependencywalker.com/
http://titaniumgl.tk/
Dependency walker shows no opengl, but does show GDI32.DLL, which I assume is to do with it.
The SW dates back to at least 1997 and I don't think any major changes in graphics were made before its demise in 2007 or so . I believe it was written in Delphi.
I'm not totally convinced that it is purely a GDI issue, as from what I've read, Vista really broke GDI, but WDDM1.1 put a lot of the hardware acceleration back again, though apparently it is now more down to what the card manufacturers choose to implement. I've seen some discussions around 2012 mentioning that ATI and Nvidia have "only recently" added support.
Maybe there are some corner cases where it is still poor.
As mentioned, there is a substantial difference in speed (like 10x) between when it does a "hidden" redraw (startup and changing display mode from outline to solid) vs when it does it prograssively onscreen (panning & zooming).
I don't know enough about PC graphics architecture to understand the process, but I suspect something to do with how images are rendered then displayed.
Other PCAD users have reported similar slowdowns, but added that turning Aero off helps, however if I turn Aero on, it doesn't seem to get any slower for me, so I wonder if there may still be some Aero related thing that's persisting even after switching to the Windows Classic theme I use.