Disable 30FPS Cap
As
Batman: Arkham Knight is a console port, its developers, for one reason or another, have capped the game's framerate to 30FPS by default. There is no way to uncap this framerate lock within the game itself so you'll need to edit a configuration file within game's folder.
Go to the Batman: Arkham Knight folder and find the "Config" folder
e.g. C

Program Files (x86)\Steam\steamapps\common\Batman Arkham Knight\BMGame\Config
Open the following file with a text editor like Notepad:
BmSystemSettings.ini
Press CTRL+F to search, and paste the following line:
MaxFPS=30.000000
Press "Find"
Now edit the 30.000000 value to set desired your desired FPS.
If you want to cap the game at 60FPS, the string should look like:
MaxFPS=60.000000