Realm lag when more than one player joins

My Realm works normally when I play alone, but the moment a second player joins, the lag starts.
When 3 or more players join, the Realm becomes extremely laggy.
Blocks take a long time to break, chunks load slowly, and there’s a big delay when interacting with anything.

We tried different devices and different internet connections, and the world runs fine in singleplayer.
This only happens on the Realm, so it looks like a server-side performance issue.

Please check the Realm performance or increase the capacity, because it’s very hard to play with multiple players right now.
Thanks.