What do you exactly mean when you say, "XML file concept"?
It all depends on what kind of transformations you are doing with the XML. Are you doing any kind of complex transistions using an XSLT?
Have you considered using the XMLReader that is a fast and readonly forward mode cursor that allows you to do your operations faster if it fits your scenario?
If you are using XmlDocument, then are you storing it inmemory for a longer duration than required?
How are you traversing/navigating the DOM tree?