The same issue just happened to me. Reality Converter suddenly stopped accepting .gltf files. I tried to remove and reinstall the application but it does not solve the problem.

I tested the last iOS Beta (14.4 beta 2) and the problem is still present. Adding nodes to the SceneView scene still hangs the app UI even when calling prepare. Sometimes, it also causes ARKit to lose the tracking. More importantly, the app freezes completely from time to time and cannot be recovered unless the app is killed. The user experience is terrible. Any help or workaround is very welcome. At the moment, the only solution I see would be to move our app to Unity...

I am also experiencing performance issues with iOS 14. Is there any bug report / open radar we can follow to receive any update on this?