Between this:
danlee wrote:
I can't switch to Postgresql
and this:
danlee wrote:
IPB
I'm not sure which is more unfortunate! I haven't had a good experience with Invision since the mid-2000s. And I'm a dyed-in-the-wool PostgreSQL fanatic

That said, the reason I prefer PostgreSQL is features, not speed (though the speed is very comparable) and I don't think that being stuck on MySQL should present a major problem for you. In fact, it's easier to parallelize MySQL than it is Postgres, so if your loads get that massive you're in a good place.
Something like a forum is a good fit for MySQL, too, because few of MySQL's limitations (combining full-text search and foreign keys, transactions and full-text, etc) matter in this setting.
Edit: Whoops, didn't notice the post date. Sorry for the necromancy, if that's frowned upon here. Conversely if y'all are okay with that I'll express no remorse for twisting the wicked energies to my unholy will.