Let's face it. PHPBB was not built for this scale. It's creaking all over the place, using old-style web design and, really, is outdated. One might say it's consistently updated. I could update Windows 95 all you want, but you wouldn't stick with it. PHPBB is Windows 95.

I honestly thought that forums were dead. There was not going to be an open source system that has a clean design, is friendly, and isn't a stupid LAMP stack.

Then Discourse emerged. It changed the way I thought about forums, literally. I've got a demo running on http://54.225.97.100, for my server. That's with 600MB RAM. Discourse is scalable, uses modern technology like Rails and Postgres, and is designed to handle the load that PMC would get. It's beta software, but worth a look.

If PMC doesn't want to get off of the sinking ship, then:
Use CloudFront and CDN every one of your CSS, JS, and main assets
Minify every CSS and JS file (Rails automatically does this)
Load balancing and/or multiple servers (kinda need load balancing for that)

It's also notable that Discourse is looking for 2 partners to test it's software on a real, live, popular production environment. In exchange, they will provide all the hosting and support you'd need, and look at feature requests.

Why would one not want to change to Discourse?
Valid question. All of these skins, servers, blog posts, mods, contests, and users would have to be moved. It would have to be converted to PG, exported, and then custom modules would have to be written in Rails. I'm not saying overlook the challenges, but weigh the benifits before you reply.

Did you actually read this far?