Shirasu Hiroyuki has published some interesting performance comparisons on
XPath engines and (Java) data models at

http://www.k2.dion.ne.jp/~hirsh/xpath/engineComparison-en.html

Discussion is taking place on 

http://lists.ibiblio.org/mailman/listinfo/xom-interest

The one-line summary is: if you want speed, don't use DOM.

Michael Kay
http://www.saxonica.com/