No idea if my theory works, but when you play the game, you got a big dense base, when you look at it, you're at 20 fps, but when you look away the fps doubles or even up to 50 fps.
Could you sort off add a "looking away" filter if you are far away enough that only will show static non animated belts with a "distance of choice" performance slider in graphic setting? Basycally how other open world games works. They don't show 5 fps static animated characters way into the distance, and neither should the game as of it's current cpu taxing state. Cause as of now, the animations and belts are too taxing for it's current "dualcore state". Please apply the "open world" popin of animations and make stuff only animate when you get close enough.
Cause what you could do is that if one belt has say coal on it, in the distance, it could show just a static non animated coal sprite standing still to indicate what's on the belt atm, and if the belt is empty, or lacking mats, it won't show anything on it.
Just a thought that could help the engine/game run better until we get proper multicore/thread support.