Your comments

Nice approach, but still instead of pallets of prefabs they can easily be structured into few folders, or one, and just thick that into white folder/s and it works nicely. This can always be fast, compared to optimization where the time will always increase a bit with the size of the project and thousands of assets. 
Connect that with the shortcut/buttons to whitelist different folders or quickly change between them and you have some speed level design tool, that would allow you to quickly place all sorts of assets/prefabs. 

I hope you will consider this even more. 

Thank you for the reply.