[Editor's Note: If you're using entities, then you have no choice. XML Catalogs can speed DTD resolution.]

Never use 

$dom->resolveExternals=true;

when parsing XHTML document that has the DOCTYPE declaration with DTD URL specified in it.

Otherwise parsing the XHTML with DOCTYPE like this one:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

will result in PHP/DOM downloading the DTD file from W3C site when parsing your document. It will add extra delay to your script - I experienced that $dom->load()'s total time was from 1 to 16 seconds.

elixon