Christian has some interesting thoughts, results and code up here, parsing a 10Mb XML document with PHP5′s XML apis. Highlights the trade off between performance and memory use when using DOM vs. Sax / XMLReader.
Personally a little surprised that XMLReader proved faster than SAX. “Pull style” XML parsers are usually implemented by “re-routing” a SAX parser and from a quick glance at the libxml2 source, it seems to do the same. I guess executing alot of callback functions at a C level is faster than executing them up at the PHP level.