Optimization is hard mid development, the biggest issue on mobile is actually memory which I think feeds into the FPS problem. The more the game expands the more memory, assets and other things loaded which seems to increase burden.
These things are fixable, but they can cause problems while things are still worked on. I can only do so much for now, but I can promise by the time the steam release comes or probably shortly after, some of these optimizations will be implemented to better control memory usage. Along with some things like broader LOD usage is also on the horizon.