Flying unexplored areas with an elytra on a long-term Realm is very laggy and not fun. You can fly for couple chunks until you get the message saying that chunks are taking time to load, please hold. This happen even when using firework rockets convervatively and mostly gliding. The problem is not internet connection or client device performance, it seems to be the Realms server itself as non-Realm worlds does not seem to have this issue. We are using the larger 10 player Realm server and it still struggles even with 2 players exploring new areas together.
This happens on my realm too but not only in completely new chunks. We get the chunks loading/you may experience performance issues message in well used zones (overworld) We've not tested this in the end (not wanting to fall into the void because of the chunk loading issue) We have pretty beefy computers and good internet speeds. I even invested in a 10 player realm to see if that would give better performance than the 2 player realm (uploading our long term world to that new realm) It made zero difference.