Well seing as about 100+ lines of code are called everytime you move, I dont think chat messages would be a big problem
On messaging someone, your message would be sent to a mysql server. Every second each server would check to see if any messages need updating
Think how much bandwith that would use, Checking EVERY SECOND, I know for a fact that the vault alone taxes the server alot.
The client isn't taxed at all by the vault. The server is though, Almost everything goes through the main database
As i said its all local to the servers. Vault uses more resources than this because it has to save an array of items.