In a new post to his blog Brian Swan explains why the latest versions of PHP (the 5.3.x series) are faster now on Windows than some previous versions have been. (Hint: updated technology can work wonders sometimes)
[Rasmus Lerdorf recently said at a Seattle meetup] "If you aren't running PHP 5.3 on Windows, you're lucky…because you have a 40% performance boost coming." He clarified this by saying that, with some help from Microsoft, improvements were made in PHP 5.3 that led to a 40% performance improvement of PHP on Windows. Because he didn't go into the details of why this performance boost was realized, I got questions in email the next day asking about why.
The information in a borrowed slide (from a presentation by Pierre Joye) shows what the differences between the versions are - things like the use of a more modern compiler (VC9 vs VC6), calls to the Win32 API directly and better library management.