So, why has Facebook rewritten the PHP runtime? Because PHP is obviously too slow for their tastes. A few years ago, I had a coffee meeting with some of the folks from Zend. When they asked what I had been hearing about PHP in the market from my sources, I hemmed and hawed, then told them that I had heard people complaining about how slow PHP was.
Other sources suggest the release won't be a full runtime but instead a sort of compiler for PHP applications including an interview with a Facebook employee who drops a hint about their future plans for the language.