So here's the jist.
download more ram here[downloadmoreram.com]

when you are ready to export the scene to Ravenfield, duplicate the scene so you have a backup. Name the original "wip" and the duplicate "v1"
use "v1" for this tutorial

1. you click "create empty"
2. name that empty "everything"
3. literally select every object in your scene (except anything that's animated) and drag it into "everything"

4. Ravenfield Tools > Map > Scan Pathfinding (or) Export Open Scene as Map

that's it!
your hierarchy should look like this^

If you want to edit your scene further, make sure you edit the "wip" scene. When ready to export again, simply redo these steps.

♥♥♥♥♥♥ if I know
My theory is that when Unity loads a scene in Ravenfield, it does puts everything together object by object. The only reason this takes time is simply due to the amount of time it takes to "choose" each object after loading the previous one. I don't know the proper computing terminology for this so it's difficult to explain.

What I'm sure about is that this is exactly the same reason it takes so long to load custom weapon/model content.

What I think happens is it follows a process of load object-next object-load object-next object.

Hence, if there is only one object in your scene, it loads that one object and that's it.