I am experiencing very low FPS on my new save, 138 hours, all available tiers unlocked. My computer hardware resources are not maxed out, GPU, CPU & RAM still have plenty of room to move. It appears to get worse the more conveyer belt movement there is, I have a mammoth belt bus system going :), the factory is also multi-level. Below 10 FPS is borderline unplayable when pinpoint accuracy is needed when constructing, fighting enemies & jetpack flying. The construction targeter has also disappeared.
This game is awesome, I hope this FPS issue can be fixed real soon!