Actually a decent net speed will do. But the most important things is latency also known as ping result.
You might have 50mbps broadband, but if your ISP (gateway, proxy, link) introduce too many latency between your PC and solarose server, you might freeze in prison which will result disconnection.
Same goes to slow net as well. In prison, you are seeing so many mobs with different activity. All these mob existance and activity information (packet) need to be send from server to your client and vice versa. If some one nearby your area, more packets need to be send. If he/they AoE, more and more packets need to be send. So, if your net speed can't handle these packets in real time way.... you may lag, freeze or dc.
Tested :
Mid to High End PC with :
(Intel Core2Duo Q6600 @ 3GHz, 4GB rams, Radeon 4850 1GB, etc)
384 kbps adsl : Almost never lag in prison. (Coz I am the only user of this broadband). No torrent at background.
384 kbps adsl : My wife youtubing at her laptop :: No lag at most of the place but may freeze in prison.
384 kbps adsl : 2x Rose clients + uTorrent at the back : Almost no lag even in prison (if i control upload speed in my torrent). May face dc if my uTorrent run at full speed.
10 mbps SDSL : Office line. Most of the time I will freeze in prison (go 39). Suspect, this line heavily congested by office usage even it is a lot faster than 384 adsl.
250 mbps T Line Connection : One of my real life job customer network net infrastructure. Lag badly even at Junon city. I can download from ftp server at lightning speed but torrents, rose, p2p suffer badly. Reason : packet shapping. Perhaps they filter UDP.
So, I would like to recommend, if you have lags or dc at prison, try to broadcast and get feedback from others, so that you can confirm either this is server side lag or your side lag. Both can happen from time to time