I like the screen recorder in iOS 12, but I was hoping that it would get more frames per second.
I don't see any settings screen. Am I missing something simple?
You aren't missing anything. There are no settings whatsoever that you can change to customize the capture characteristics of the built-in iOS screen recorder.
However, a simple Web search reveals a bunch of 3rd party iOS/macOS/Windows screen recording apps that you can try out to see if they offer better quality/cutomizations.