MyBB Community Forums

I know that forums would generally load slower, and a shoutbox would be borderline unusable, but what benefits are there to housing the forum database on an external server?

Would it make sense to house the forum files on a DDoS-proof server, and the database on a lightning fast external server? Could attackers still gain the IP of the external server?

What do you mean by external? If you have a very large forum then it may be beneficial to host the database on a separate server, but given it is on the same network there will be essentially no speed difference. Hosting on a database somewhere else on the internet would make the site insanely slow, I can't really think of any benefits it would bring.