My troubles arose in the process of migrating a file to a new template. I never did fully solve the problem and decided to abandon the new template and continue with my existing one.
I did, though, learn a few diagnostic procedures. Perhaps they'll be of use to you:
Download the "Check Duplicates" tool from the GS site and use it to see if you have inadvertently superimposed duplicate elements on your plan.
Turn layers on and off and see if any particular one causes a slowdown. If so, investigate the various classes of elements present on that layer.
Use the Marquee tool to show only certain portions of the project in 3D, and see if it's only one portion that's slow.
Do a Save As and and use the multi-story Marquee to select and then delete entire classes of elements, one at a time, checking for speed.
Good luck!
Ray