If the project is big in terms of the total area, you can reduce lightmap resolition either for the whole scene (the editor Bake tab, Lightmap resolution setting), or only for the largest objects in the scene such as terrain or walls (the Objects tab, Custom lightmap resolution setting).
If the geometry has too many triangles, you can identify the most complex objects in the Objects tab - the objects list in this tab is sorted by the number of triangles. Shapespark doesn’t include geometry optimization tools, so you need to optimize or replace too complex objects in your 3D modeling program.
Could you share a link (here or by email to support@shapespark.com) to a scene that you need to optimize, so we could provide more specific hints?
One way to present multiple walkthroughs is to embed them together on a single web page, each walkthourh in a separate iframe (see the iframe HTML code).