Cutting your scenery into pieces with only one being rendered at a time. You can just toggle invisibility on the different sceneries if you want to do it in one project. But either way, I recommend upgrading your specs if you're looking towards good framerate with larger schematics. If your processor is above 3Ghz and your ram is 8 gb or more, all you really need to focus on, is getting a better gpu. I mean... My processor only runs at 3.4 Sure I may have 16 gb of ram, but memory isn't the greater issue in mineimator, it's the gpu. I have a 1080 wich obliterates any schematic you throw at it.
I'm not kidding:
And of course I'm not saying you should get a 1080 but a gtx 1060 with 3gb of vram is fairly cheap and gets the job done if you want something more budget friendly.
Either way, it's up to you how to tackle this problem