Ehm, you can simply read the thread or my last post, that would explain it.
Render instances: Like a copy of your geometry in the scene. You still have the source geometry object in your scene. Which makes the scene slower and the scene file bigger.
Proxy: You bake your geometry and textures into a file outside your application. You only have a placeholder in your scene. So the scene file is not getting bigger and the viewport is perfectly fluent. Orbx already does it. But as i said:
1) You can not change materials of Orbx objects without recreating them
2) Creating them takes ages.
3) File sizes of Orbx objects are way too big. My library would take 3 times the space, if i would use them.