Networking Improvements in On Topic
Since neither your bandwidth nor cpu are being maxed out, I have to believe you are getting lag spikes (where your cpu is instantaneously maxed) rather than a continuous lag. This may occur when some world event forces updates on all users simultaneously or something to that effect. Does this seem likely, given what you've seen of the game, or are things basically sluggish all the time?
To properly diagnose this, I think we'll have to host a busy server from a debug version so we can see where the bottlenecks are. But any further specifics you can provide would help. Like I said, we can run some simple tests to see how we do (vs, say, FTP) in mostly idle-states and work from there.
The games have occasional lag spikes, but I don't worry much about them. Every online game I've ever played has its lag spikes every now any again, I don't consider it out of the ordinary. I'm more worried about the continual lag buildup as player count increases. 30 70 and 90 seem to be main lag increase points. Before 30 players the game will play as lag free as it will get when connected to a BYOND server. At 70 the lag will start to get noticeable. At 90+ players in HU I'd say the game is nearly half as responsive as it should be.
I run my games in debug mode if that's what you're referring to, any steps I should take to get you additional information?
Have you tried increasing your tick lag to see if that has any effect at all? Even if it's not a direct improvement, if the behavior is different it might give us some insight into the problem. For instance, it might worsen the behavior for less players but improve it for more.
From the developer's perspective, the best thing to do to accomodate large volumes of players is, as others have mentioned, using link() and multiple servers. In recent updates we should have removed any of the old prompts that bugged players when they were moving between servers, so the experience is seamless. Correct me if I'm wrong about that and we will fix it.
EDIT: That's with 5 servers of HU and a server of PC being hosted from DDs.
Also, I don't have a multi-processor system, I have a single-processor multi-core.