In Seafile the server doesn't notify changes to clients, instead the clients poll every 30 seconds, which means that changes are synced with a 30 second delay (and presumably bandwidth is wasted). See https://github.com/haiwen/seafile/issues/992. I'm not a fan of Dropbox but at least the lag is only a few seconds, which I think is because the clients notice changes with inotify and send them to the server, and the server probably pushes updates to the clients.

I haven't tried to use it seriously, but seeing that it doesn't do this right discourages me a bit from testing it further.