You have spent a lot of time optimising your sculpts and you are cloning most of your environment, and the graphics thermometer is keeping nice and low, but your Gameplay thermo is going crazy high and you haven’t even started on any logic yet! What is going on?
The chances are you have grouped your sculpts and are cloning groups. This is normal for tiling things like floorboards – you take a small piece of floor, clone it to make a strip and group that, clone the strip and cover the floor. I made a wire fence using a small piece of wire that I cloned into a shape and then a bigger shape and so on. However, grouping adds to the gameplay thermo, and constant cloning of groups will add a percentage every time you do it.
If you find your gameplay thermo is rocketing all you need to do is to ungroup your cloned objects. Make sure you have your landscape pieces where you want them first, and then select and ungroup. Your thermo will rapidly go down.