This fix has been tested with the Steam version of the game, but should work with any client's version of the game. Unlike AC Brotherhood and Revelations, this game has a 2 part fix, so please follow the instructions thoroughly.
NOTE: I have not tested this fix with the standard, non-remastered version of AC3, so I cannot speak for how well this will work for that version.

1. Launch AC3 Remastered through Steam or whatever client you are using to play.
   
2. Press ALT+TAB to return to the desktop.
   
3. Open the task manager using CTRL+SHIFT+ESC.
   
4. Head over to the "Details" tab.
   
5. Right click the executable, typically towards the top of the list if sorted alphabetically.
   
6. Click "set priority."
   
7. Change the value from "Above normal" to "High."

1. Right click the executable.
   
2. Click "set affinity."
   
3. Uncheck ALL Cores except Core 0.
   
4. Click "OK."
   
5. Right click the executable again, and click "set affinity" once more.
   
6. Re-check ALL CPU Cores.
   
7. Click "OK."
   
8. You should be able to close Task Manager, and ALT+TAB back into the game, but now with even smoother and more consistent performance!
   
   PLEASE NOTE: THIS FIX MUST BE DONE EVERY SINGLE TIME YOU LAUNCH THE GAME!
   Hopefully this guide has helped you solve any issues you are having with stuttering and/or low FPS in a game that has no right performing poorly in 2020. I would appreciate you listing this guide as a favorite and leaving a 5 star review if it did help in some way. Please check out my guides for AC2, AC Brotherhood, and AC Revelations, as these games have similar fixes for their performance/stutter issues.