Realm lags severely, then rolls back as a specific player joins

I've seen many issues where people have their realms randomly rolled back, but THIS is different. Usually, me and my friends play and we never come across any issues, until one of two players joins the realm. Whenever the players, JephthaKing and the owner of the realm, MasterKian7698 join the realm, the game instantly starts to lag like crazy to the point where it is unplayable. Then after that, the realm is rolled back and lots of progress made is reverted. I have a feeling it is a problem cause by them but they have no knowledge of it or why it happens.