Flutter vs React Native

React native (or flutter too) works best with list like content, where items can be “re-used”. If you have a lot of network calls, or a lot of complex views, with animations, transparencies, big images etc on a single screen, the performance will suffer. RN uses a JavaScript bridge to communicate between the UI (main thread) and logic (JS thread). Think of it like a single server using a single tray to serve food from the kitchen to the tables. If you have more food, it’ll be slow. If you’ve one customer ordering too much, it’ll be slow, and stressful for the one server. There are performance considerations of course, like you can make sure the folks who ordered food before don’t get served in this round of the server’s trip. But still if the food ordered is massive (design is too “fancy”) the performance will be laggy.

IMO, flutter is better than react native since it's got better performance, ahead of time compiler therefore smaller app bundle. My assumption is that the code might not be optimised, since flutter does support native gestures and ui + for a simple fantasy app (and not some heavily developed app), flutter should work seamlessly without lags or jitters. If you're at an MVP stage, consider flutterflow, works great and they build optimised packaged code. If not MVP, consider reworking the flutter code in order to optimise it and write good quality code.