On his blog Bas De Nooijer has put together some PHP Solr benchmarks around a few different ways to handle the response data.

Solr supports multiple output formats. Some are for general use (xml, json) and some are even language specific. If you're using PHP these are the most logical response writer formats: xml, json, phps (serialized php), php (php code to execute). On top of that PHP offers multiple ways to parse XML. I'm benchmarking these options to determine the most efficient decoding to implement in the next major version of Solarium, but the results should be useful for any PHP Solr implementation.

He includes a snippet of code he's using to generate the benchmarks, a PHPUnit test that pulls in two different json-based results and, for now, just runs a json_decode on it. You can see the results of his testing here, graphing out the different response handling methods and their time (in milliseconds) to parse the response. It's interesting to see that there's a direct relation to the size of the data set and how long it takes for the methods to execute.

The current code for his benchmarks is available over on github