MacNN and AI share the same server for UBB, which is used exclusively for that purpose (minus our mail server, which also runs on the same box but consumes less than 1% of the CPU).
AI forums are very active--too active, in fact, at times. We notice this immediately because our mail servers becomes very slow to respond, as are the forums. Rather than leave everything up and running slowly (compromising both our mail and all of the forums), we shut AI's boards off. Many members of the AI community seem to be constantly refreshing numerous pages, using the boards as a real-time chat arena.
For those of you familiar with [Red Hat] Linux, performing a load check (w) on the forum/mail server typically yields a figure of 10 or less (this should really be under 5, and ideally under 2). At times, like during Macworl when everything gets bogged down, this figure jumps in excess of 100, sometimes approaching 200. That's ridiculous, and that's when we turn things off.
We are currently evaluating our options, including dedicating a box exclusively for AI forums. We hope to have a solution for you soon...