It shouldn't be non-native, that's the problem. The best apps are always native ones. They make the most of the available APIs, fit in the OS the best both graphically and technically, and are the fastest.
If it has to be cross platform, something like React Native which leverages system controls would be a better approach than flutter which draws its own controls. But even there you will have to deal with a lot of translation due to it being JavaScript.
Are you proposing that iOS might be so broken it's impossible to flawlessly copy a handful of screen-size rendering buffers at 120Hz? Because if you can do that, then it's possible to make a flawless no-lag non-native 120Hz UI.
I don't know if you can even access the screen at such a low level on iOS, I think for sure you'll have to go through a middleman to get pixels on the screen.
Do folks have some examples of non-native iOS platforms that do have flawless no-lag 120Hz display?