Batman: Arkham Knight's PC version comes with an enforced frame rate cap of 30. Thankfully, you can increase the cap by editing a single configuration file. Below I will go over how to do this.
1. Locate your Steam folder, which is typically under "Program Files" or "Program Files (x86)".
2. Head to "SteamAppsCommonBatman Arkham KnightBMGameConfig".
3. Right click the file named "BMSystemSettings.ini" and choose to open the file with Notepad.
4. Locate a line that states "MaxFPS=30". Change the 30 to 60.
5. Exit out of the file and save your settings.
Next time you load up your game the frame rate cap will be 60 instead of 30. Enjoy!