Since iOS 10, there are some apps (such as Clash Royale, Google Maps) that are stuttering. This issue has a long history, first in iOS 10 it only happened when my finger was on the screen (in this case, the app instantly dropped the frames to 25FPS). As soon as I released my finger, it went back to 60. They fixed it in iOS 10.3, then iOS 11 and iPhone X came. With that device, the same issue was happening, but ONLY with zero brightness. Starting screen recording has solved the issue or increasing the brightness. This has been resolved as well, in iOS 13. Now in iOS 14 (beta 8) the thing is, the issue is still here but in a different form - the game has a "News Royale" section which will start stuttering (25FPS) if you go out of the app, and go back (most of the times, it's not happening always though, sometimes it requires more than one try). I have made a video which clearly shows how to reproduce the issue.
Please report this issue to Apple, so that the affected apps would behave correctly. After all, we would expect a smooth experience from our phones if they are that expensive, right?