Pardon my ignorance here, but Rasmus starts talking about latency and concurrent requests about 16:30. He then mentions "one line of code" installation of something that sounds like "aqua code cache" that gives WordPress a 3x performance boost in response time / number of transactions per second. Anyone knows what he's talking about there, shoot a note and I'll update this permalink with the info. He goes on to talk about much more hardcore-PHP-nerd WordPress optimization stuff.
The scoop via Joost Schuur:
Rasmus is talking about an 'opcode cache'. It essentially caches the parsed PHP scripts in memory. That means PHP doesn't have to read it from disk or even convert the humanly readable PHP script code into executable machine code (opcode). The one he's referring to is APC, which is likely going to be included in the next big release of PHP. Xcache is another popular kind. I run it on my site and got about a 3x performance increase on page request server times, so the numbers add up. The W3 Total Cache WordPress plugin has the option of using Xache or APC to keep popular pages in memory via opcode caching.