WebCamTexture FPS faster than Application.TargetFrameRate,WebCamTexture at higher framerate than TargetFramerate

I want Application.TargetFramerate to be lower than WebCamTexture.requestedFPS. For example, TargetFramerate = 5 and FPS = 30.

When I've tried this, both in the editor and on iOS, WebCamTexture cannot update faster than the Application.TargetFramerate.
However, when using ARKit, I can use ARCameraManager.frameReceived to update faster than Application.TargetFramerate.

This tells me I'm not hitting a fundamental limitation of Unity, but an implementation quirk of WebCamTexture. Are there any known workarounds?

For a more common example, consider trying to record at 60fps on iOS, where targetFrameRate defaults to 30.