After a certain amount of days, for me it's around 30+, going anywhere indoors (ruins, houses, shops, mant hive) my FPS drops extremely from 60 fps to lowest 5-20. I saw somewhere that posting a profile might help? I'm not sure really.
Right now my current save is on day 55.
Maybe it's because I've explored almost all of the ruins entirely?
My computer's specs are decent, if not better.
AMD FX(tm)-8350 Eight-Core Processor
AMD Radeon (TM) R9 380 Series
Play WX, go about 40 days in, explore several complete ruins