In order for Rhino to shade the NURBS surfaces (i.e. show anything else than the edges and isocurves), it needs to create a render mesh. This is not accessible in a direct way but you can influence how it looks by changing the settings in
Document Properties > Mesh. You can also extract the render mesh. When these render meshes get heavy (either because there are many objects or the settings dictate that lots of triangles are created), it will take some time for Rhino to generate these. That is why these are saved in the Rhino file (so that they don't need to be created all over again when you open the file the next time you will use it). You can use the command
ClearAllMeshes to delete these meshes.
Nope. At this point, Rhino 6 is a WIP (Work-in-Progress) for the Windows platform. After it gets released for Windows, work will begin to make it release it on the Mac and that will also require a WIP phase. The code base will be the same for the two versions, so Make2D will also be faster on the Mac at that point.