I'm not a database specialist myself, but working in the ICT business I know that databases will need heavy maintenance.
It's possible the problem is in the database.
I don't know what kind of Database Steve uses, but i know that there can be problems due to the number of connections to the server or the actual threads on the server where the database is running on.
small example:
If you have "notification on" for a topic then every reply on that topic will produce an email to all the people who want to be notified. this wil produce hundreds of emails. this will effect the performance of the server.
but then again the problem could be an other 1000 things.
looking at the number of 1156 users, the impact of the "havy users", all the topcis, reply's and posted attachements I think this database is growing to a "proffesional" size.
I reccon that a medium/big size company has employed a fulltime database programer who will have his hands full of this kind of database.
this forum is free, so lets give steve some time to solve the job, believe me, this can be a difficult problem!