Whenever this happens, I'm all for archiving it.
I know you've explained this, but I think I didn't understand the explanation. What's the problem (in terms of performance) in making this forum read-only and then starting afresh?
I mean, I realise the archived forum would still have the same performance issues but surely it would be used so little as to not matter? It would be used as much as people use the search function currently, at most. Which for me, is very very seldom. Regardless, the overall load should be much lower shouldn't it?
If for some reason that's not practical, how about dumping this database, getting someone to host this forum read-only somewhere and again, starting afresh here. The archive might fall over a bit but... better that than this place suffering. The slowness is not the fault of the software or whatever, just the size of this place. Butif it's keeping people from posting then we need to make sacrifices I think.