On his Adobe blog, Mike Potter compares three technologies for communicating with their Flex product - JSON, XML, or AMFPHP.

I've created a small performance study that shows time to load three different types of data from a PHP backend... Data transfered as PHP objects from AMFPHP, data transfered as XML and data transfered in JSON notation.

He ran his tests with the AMFPHP library and the Zend Framework (for JSON). The XML is simply printed out and pulled in.

The results of his test showed that, up to a certain point, the XML and AMFPHP were neck and neck (with JSON lagging behind). Once they reached a certain point, though, XML powered ahead, blowing past the AMFPHP library. Of course, just the sheer speed isn't all there is to an application, he reminds us - usability, serializing data, connection type, etc all need to be considered as well.