Game freezes

After a certain time in a new world the game freezes. You can read the text "saving world" in the right bottom corner of the screen. I could pinpoint it to a lack of provided RAM. The default 2 GB RAM is not enough, even 4 GB does not work. If I allow 8 GB RAM (Java property mx8GB), the game works as intended. I think this is a bug because you could run Minecraft with 2 GB RAM until now.