It’s not about server but shards now.
Shards are in fact instances of the world on virtual server.
Some of the lags are related to your computer hardware and internet connection.
I suspect that, like in battlegrounds, the game engine select one player to send all data to the server so every actions accross all players interacting can be synchronized (instead of having 20-40 players communicating with the server). Because the people die fast in wpvp (and rez far from the action), the game engine is always in player selection mode. Not everyone have a good internet connection. If the selected player have a bad internet connection then all will lag.