If you have a lot of save files and it's slowing down your game, you can remove save files from the game by turning off Steam Cloud. Then you can go to your Settings in the menu and click on View Game Data. Open the file (their names are based on the name of the level; i.e. "level1" , "the_library") and remove the .sav files.

Another alternatively, you can access from your user account directory and add the following line to the address search bar:

AppData\Roaming\Godot\app_userdata\Lingo\level1

You can then remove these files from the respective folder.