I don't think the network side is the issue in almost any MMO when it comes to the client. The amount of data that is sent and received is tiny. It's most likely the number of draw calls that is the performance bottleneck. A high end machine in 2018 can handle around 2000 draw calls per frame (remember, this has been single threaded until recently!). You could easily have a single draw call per player, sometimes even more. That alone could take a system to its knees in an MMO with lots of players.
