Matthew Turland has this new blog post looking at some benchmarks he's generated for a group of mainstream PHP HTTP clients:
One of the interesting bits of research that I've done is benchmarking various mainstream PHP HTTP clients. Of course, we all know that there are lies, damned lies, statistics, and benchmarks, so take these with a grain of salt.
He ran them on his Sony Viao on Ubuntu with a stock PHP5 package. The tested packages were the pecl_http extension, the streams http wrapper, curl integration into PHP 5, PEAR::HTTP Client class and the Zend_Http_Client component. He includes the code he used for both a basic request and for something slightly more complex (posting form data). He used the XDebug and KCachegrind combination to produce the results.