Hello! There could be a lot of reasons this could be happening. Please try out the following:
1- Check your PC specs. These are the recommended specs, which can be located at the game description if needed
RECOMMENDED SPECS:
- GTX 750 /AMD 6870 or Better
- i3 12100F / Ryzen 5 5600 or better
- 4GB of Ram or better
- SSD (Optional but good for quick loading)
2- If your specs are fine, Run the Benchmark tool located on your Items, and let me know what happens during it.
3- If after these steps, you are still having problems, do link/send me your save file for further inspection.
Make sure you are running the latest version of the game, and try to see if the problems persist even on a new file, as sometimes, old files might not run well on newer updates.