What are the FPS of the FaceTime Camera for the iPhone 6S vs 7+ vs X vs iPad Pro? We are capturing video from iOS devices and processing it using OpenGL but are encountering a FPS difference between the devices when trying to set them at their maximum rate. 

The iPhone 6S and iPad Pro FaceTime cameras are maxing at 60 fps.

The 7+ and iPhone X however are maxing at 30 fps. 

It seems unusual that the FPS would decrease with later versions of Apple's hardware. We are trying to figure out if this is a software issue or simply the technical specifications for the frame rate for the FaceTime camera. 

We looked on Apple's technical specifications page but the frame rates of the FaceTime cameras aren't listed (though the rear cameras are.) For example:
https://www.apple.com/iphone-6s/specs/
https://www.apple.com/iphone-x/specs/

What are the FPS of the FaceTime Camera for the iPhone 6S vs 7+ vs X vs iPad Pro?
Here is the beginning of an answer.

We found a private library method of Apple's (not allowed in the app store) called CAMCaptureCapabilities which has a read only property called front1080pMaxFPS. 

We were able to run this on an iPhone X and found the front camera maximum to be 30fps. 

On the iPhone 6S/6S+ we have an app that is able to set the camera to 60fps. 

We still need to test on other phone models to see their maximum (i.e. particular iPhone 7 and 8.)

Interestingly, this means the iPhone X's front camera is slower than an iPhone 6S/6S+.