When in Windows Media Player, it will not be adding files to the database, it's slow on startup, or it's just laggy in general. Do the following steps to rebuild the database in Windows Media Player, note this will be done automatically.
To Resolve: Edit
1. Exit Windows Media Player.
2. Run - %LOCALAPPDATA%\Microsoft\Media Player
3. Select all the files in the folder and delete them. You do not have to delete the folders that are in this folder.
4. Restart Windows Media Player.