Turns out, old server-side technologies are not designed with real-time user interaction in mind. They are good at serving massive amounts of pages and have caching and scaling technologies to cope with increased load, but they are not really good for having constant back and forth communication with the user, which requires passing small amounts of data every time at a low latency.
Enter Node.js. It was developed by Ryan Dahl and made waves in JS community last year. Node.js essentially implements the entire web stack with an event-oriented philosophy in mind. No more threads, no more I/O bottlenecks reading from file and database connections, it's all callbacks. As in your browser.
Who knows, maybe in 5 years there will be a massive migration to frameworks based on Node.js. Just like there is a massive migration from static webpages to rich web apps today.