It's hard to say because most of the time when you are in the batmobile your frame rate is in the 70's to 80's most of the time.
Running the game on a SSD would help but I don't think it would be the sole cause of your issues.
I did a quick Google search and found multiple forums with threads about low frame rates when in the batmobile, I even found one on the overclockers.com forums which I suspect, is your thread.
http://www.overclockers.com/forums/...ham-Knight-frame-drops-Ram-or-HDD-issue/page4
Then I found one on the Steam Forums of multiple people complaining of the same issue.
https://steamcommunity.com/app/208650/discussions/0/520518688945849324/#p1
On what I suspect is your thread on the Overclockers.com forums, you mention that your GPU usage drops which is something you have not mentioned here.
One thing I would suggest trying is going in to the nVidia control panel and finding the Batman Arkham Knight profile and setting the power management to Prefer Maximum Performance to see if that helps with the GPU usage, drops but I cannot guarantee that it will.
I don't want to sound rude but this is not something you are really going to be able to fix, but you could try some of the things mentioned in other forums like turning off Smoke and Motion Blur and what I mention above.
The game just seems like it's really buggy and doesn't play nice on the PC which is why I have avoided buying it, and to be honest seeing this thread and multiple others around the net just shows that you could have 2 pc's with identical, greater or lower specs and they would still suffer from frame drops either in the same spots or different spots.