I put up a test server with a tiny map a few days ago. This was to stress test the game by forcing everything into the same area, meaning that when you spawn the server sends you everything. Amazingly the server didn't blow up.
There were issues when spawning though. These still exist to an extent but now they're manageable. Half of the problem is that when you spawn into the server you're being sent information about every player that has joined the server whose body hasn't been destroyed. Right now if you join you're being sent 11,000 sleeping players, which you then spawn. Unity's prefab spawning stuff is notoriously slow. Things have got a little better in 5.0.. but it's still going to take a few seconds to spawn 11,000 players.
That said, the whole point of this was to identify potential problems. Now we know them and we can look into potential solutions.. like spawning things that are closer to you first, then slowly spawning everything else over a number of frames. For every problem there's 100 clever solutions, this is why we love what we do :)