WebCamTexture provides 30 FPS maximally for the Rear Camera on iPhone XS Max regardless of Application.targetFrameRate, although this camera supports 60FPS, 120FPS and 240 FPS.
Is it a Limitation of WebCamTexture or Bug?
How to set 60 FPS on WebCamTexture?
Update: I released Camera Feed on Background — Unity Asset which operates WebCamTexture class and does all the work for you.