My save files are completely screwed up. Played for the first time on normal with all graphic settings set to epic since my pc can handle it, and everything was fine until day 8. Then i noticed a heavy frame rate drop, but changing to the lowest settings didn't help. Tried to switch to sandbox mode and the game froze. Restarted the game and my save file was literally unplayable, so i deleted it. Restarted on easy and proceeded until day 3, then i saved to quit. The next day my newest save file was back to day 1 with all my stats resetted, and my older file was back, with a clone.
First: the file coming back is likely based on how the save was handled - if you had multiple versions of it, at any time, - to completely start fresh, I would go to the game folder, and delete the slots along with the saves - Slot file stores the header info for the game to associate with the new save files, if it says a save slot exists, it exists, if it doesn't, even if the file is there, the save doesn't exist to the game/engine.
As for lag spike - If you join the discord and dm me, I can help you out with that, when the time arises, as well as the other thing you mentioned, I can tell you what contributes to the lag, and how to mitigate some of it, and also what may be contributing to the other thing (more or less speculation)