XmlElement is a subclass of XmlNode (and so is XmlAttribute, XmlText, XmlDocument, etc.). There is no performance implication of using one over the other at all, since XmlNode is simply the base class for all types of nodes in a DOM Xml document instance.
It just so happens, that many of the APIs in the DOM return an XmlNode reference because they could return several kinds of nodes. For example, with SelectSingleNode(), your XPath expression could return an element, an attribute, an XmlText and what not; whether you handle it as an XmlNode or cast it to the right type is just a matter of how you write your code but won't affect performance at all.
XmlElements are a type of node. In fact, if you look at the members of XmlNode and XmlElement in the .NET Framework, you will see that they are very much alike, but XmlElement has more going on. It inherits XmlNode and then is further customized. This is because an element is more specialized. A node is more general in scope. The document is a node, a processing instruction is a node, and so forth. Elements are different. If you look at the XmlNodeType property of an element, you will see that it is Element, one of the many types of nodes you find.
Element是Node的子集，XmlNode表示一个节点，包括XmlElement(元素)和XmlAttribute(属性)等。 如：
<Time> //node
StringValue //node
</Time> //node
</Alarm> //node
以上Alarm(元素节点),lock(属性节点),Time(元素节点),StringValue(文本节点)都是Node，但是只有 <Alarm>......</Alarm>和<Time>StringValue</Time>是Element