They are two separate connections going at the same time, and they each deal with different aspects of the game. Home sends/receives data about your chat, guild info, item tooltips, auction house. World sends/receives data about your spells, abilities, character/NPC movement, combat mechanics.
Both of the connections together make up WoW as you know it, and in terms of latency it is possible for one connect to be going to one data center and another to be going to a different data center. It is also possible to experience an issue with one connection and still remain connected to the other. This is why we sometimes see reports of players not being able to cast or see other players move but still being able to chat. Their home connection remains intact while their world connection has dropped or is lagging.
Nazjatar is a Central realm while Suramar is a Western realm, so you are correct there. If you are connecting from the Eastern part of the United States you would likely see lower latency on Central realms, due to decreased distance and better routing to that data center. However that is not always the case as routing can change or the router may become congested, as we have seen previously with some ISPs.
Hope that info helps!