Laggy geometry window

[micheus]

I see no instruction on how to fix this, which is how I typically make walls, but at a later time remove the holes.

Maybe because that is produced by you intentionally.
When you removed (Dissolve) the faces from the wall thickness Wings3D will create a back face for the other faces (like in this sample) - In truth, if you select this object in body mode [B] and use the Separate command you will get two objects. Wings3D always works with a manifold object.

But, to fix that you don't need to separate them first. In face mode [F], just select each opposed internal face and use Bridge command. If you have another objects that have more than one face (different of the one you shown us above), then you must make them one before use Bridge command.
If you already used this command before, you probably understand what I'm talking about. Otherwise, let me know and I can make some pictures.