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+.