I'm sure the back-room-boys who are beavering away developing Octane have a few good ideas up their sleeves to improve it
There is limit to how much can get made into an OBJ without it getting massive, and 'instancing' might help.
A SKP with millions of polygons IS enormous!
Currently you must manage you SKP contents to export just what's needed for the make the render.
Use layers to hide unwanted things.
Be frugal with your modeling and don't include internal facets in assembled objects that will never be seen.
Also simplify objects that are to be far away [e.g. check out my MatrixProximity tool at SCF].
Hidden objects, or those on hidden layers, aren't exported - but they have to be 'inspected' to check their visibility state, so the process will be slower on large models even with a lot of things 'hidden'.
If you are rendering the outside of a house you don't need to export the bathroom fittings.
If you are rendering the bathroom you don't need to export the roof or the garden modeled etc etc
If you want blades of grass use a few near the camera and model the middle/far distance grass as a texture with a bump-map as you'll never appreciate it otherwise anyway.
Don't use distant trees that have leaves and twigs that'll never be rendered or seen
But I'm sure you know much of this...