3/4 of all the available save slots?
have you tried completely reinstalling the game and erasing the AppData\Local\MANTIS folder?
Viewing post in Femdom City M.A.N.T.I.S. [0.8.1] First Endings Edition comments
Yup, almost every available save slot.
I erased this appdata folder and unzipped again the whole original folder. The games runs smoothly, if I don't save often. I am in 20+ days when it started lagging again.
Also, there were no install process at all, just running the game.exe and the game starts immediately.