Anything over 50 forums reeks of an information architecture problem. Less than 20 is probably more ideal for visitors. I'm guessing you have some sort of dynamic content that you are creating forums (or sets of forums for)... This is what tags (self-building) or prefixes (predefined: x forums * y prefixes) are for.
I'm not trying to rag on you... we've just been there, and it always ends horribly wrong. Most forums have the forums cached, and the complex permissions data structures are also cached and stored per forum.
Even if you survive the short to medium to performance issues, they'll never work when you hit any sort of critical scale, but you'll give up long before then with maintenance issues.
BTW - I asked a similar question on Discourse -
https://meta.discourse.org/t/content...ations/12184/2 - the answer wasn't very helpful, but I think my post above is the real answer.