App lags even on iPhoneX

Hello

I recently tried to test my app on iOS and noticed that even on iPhoneX it lags.
When I run it on a very old and cheap (100$) Android device (below version 4.4) it works like a charm.
So I don't believe that it has something to do with the graphics (poly-count < 500).

In the app you can rotate the scene with your finger. May it be that the touch-position on iOS isn't accurate?
Sorry for the little information, but I don't know where to begin with my search...

So maybe anybody already had a similar problem?

Thanks!