Explanation: This version of Unreal Engine is simply not good for open world games. Developers which use this engine almost always make the same mistake and set the numbers too low in the configuration files for some reason and Rocksteady is not an exception. All of the fixes below can pretty much be used for any game running on this engine to deal with the stutter or fps drops when exploring the game world. Keep in mind however that some things like saving when entering new areas can impact it regardless, but this will also make that have much less impact.

Just know that regardless of this fix, sometimes running on an HDD will make the game slow down occasionally no matter what you do. Though personally on my HDD it runs just fine.

Fix: Perform the following changes with your game OFF.

You can find Arkham City's workable files in users/yourname/documents/WB Games.

In BMGame go into Config and open the BMEngine.ini.

CTRL+F to find "PoolSize" and set the value to whatever you want, this is the VRAM used for texture streaming and it's originally like 500 something which is abysmal. A safe bet is to set it to 4096 which translates to 4GB.

Once again CTRL+F to find "SizeOfPermanentObjectPool" and change the 116 to 232. Keep the zeroes.

Lastly find and set HysteresisLimit to 0

IMPORTANT: Due to this game having PhysX, opening the game after this point will reset your BMEngine.ini. Across the internet guides will say to disable PhysX. You can keep it on actually. Just set the .ini file to Read Only in properties to cockblock it.

Easy