technoforum.de: I'm sorry, what are we ignoring?
We have stated over and over, in public, that the more members you have and the more topics are in a forum, the slower things are. We never advise keeping more than 5,000 topics in a forum, and never advise keeping more than 20,000 members. Even 10,000 members is sometimes pushing the limits...
The same would be true for the number of forums, but that usually doesn't become an issue unless there are 50 or so... we had a case of a guy with 700+ forums the other week. Performance was horrible, and we convinced him to drop it to only 150 or so... Only, heh.
If your server is getting thrashed under Classic, then it's time to move over to Threads, plain and simple...