For examples, maXML and Noel's XML module takes 25 seconds to load a 15meg test xml file
That's strange.. I don't know about cower.xml (since it seems to crash no matter what XML file I try it with), but MaXML 2 should be able to load a 15 MB file in less than 3 seconds (at least on my computer). Maybe you're forgetting to turn debug mode off?