In my testing, the new server that rshaug contributed represents roughly a 54% reduction in page load times using the same software configuration as on the current production server.
That alternative technology stack I was talking about, called HipHopVM released by Facebook, represents an additional 66% reduction in page load times, at least based on my initial testing here.
Average time on our current server to load the forums thread index: 6.27 seconds.
Average time to load the same page on the new server: 2.85 seconds (using the same software stack)
Average time on the new server using HipHopVM: 0.964 seconds.
This is without any changes to my code.
Getting everything working on the new server with HipHopVM is going to take some time. It's a very early stage technology and the documentation is virtually non-existent.
But these are very promising numbers especially if they pan out across the rest of the site. Currently, I only have the one forum thread index page working in HipHopVM for testing. I have some work to do yet to get the rest working.
I must say, I'm incredibly impressed with what Facebook has accomplished.
(edited: my numbers were a little off)