Certain users cause the server to consume all available ram immediately when they log in.
As discussed in this Reddit thread.
I have a linux box sitting on my desk on my home network. My wife and I can connect and play together without issue, but as soon as my Mother joins the server the Java process consumes all available ram within 10 seconds. (I can open top and watch the ram statistics.)
The problem does not always occur, but happens most of the time. It doesn't matter whether she is the last person to connect or the first.
Might be related: My mother lives a few thousand miles away, and has high latency as a result.
If you would like any more info on this please let me know. I'm a software developer and would be happy to help.