XPath XPathNavigator.Select() query not performing well in a big loop

I have an XML file with 500,000 records and the structure is as follows:

<files>
    <file>
        <fileName><![CDATA[D:\Inetpub\wwwroot\default.html]]></fileName>
        <lastModified>2011-07-01 14:06:25</lastModified>
    </file>
</files>

To select a particular record I'm using a XPathNavigator with the query:

xpiOrg = xpnOrg.Select("/files/file[fileName='" & strFileNameCurrent & "']/lastModified")

xpiOrg being a XPathNodeIterator. My aim is to get the 'lastModified' value where the 'fileName' equals the variable strFileNameCurrent, which itself is populated from a XML file (500,000 records) in a loop. Is there any way to speed up the XPath query? Are there any quicker ways to get a matching record from on set of XML from another?

My entire code is like so:

    Dim xtrCurrent As New XmlTextReader("C:\xxx\_files.xml")
    Dim xpdCurrent As New XPathDocument(xtrCurrent)
    Dim xpnCurrent As XPathNavigator = xpdCurrent.CreateNavigator()
    Dim xpiCurrent As XPathNodeIterator = xpnCurrent.Select("/files/file")
    Dim xpiCurrentChildren As XPathNodeIterator

    Dim xtrOrg As New XmlTextReader("C:\xxx\files.xml")
    Dim xpdOrg As New XPathDocument(xtrOrg)
    Dim xpnOrg As XPathNavigator = xpdOrg.CreateNavigator()
    Dim xpiOrg As XPathNodeIterator

    Dim strFileNameCurrent As String
    Dim dtLastModifiedCurrent As DateTime
    Dim dtLastModifiedOrg As DateTime

    Dim sbNewFiles As New StringBuilder
    Dim sbModifiedFiles As New StringBuilder

    While xpiCurrent.MoveNext()

        'get fileName and lastModified from current node
        xpiCurrentChildren = xpiCurrent.Current.SelectChildren(XPathNodeType.Element)
        xpiCurrentChildren.MoveNext()
        strFileNameCurrent = xpiCurrentChildren.Current.Value
        xpiCurrentChildren.MoveNext()
        dtLastModifiedCurrent = Convert.ToDateTime(xpiCurrentChildren.Current.Value)

        'did it exist in the org xml list?
        xpiOrg = xpnOrg.Select("/files/file[fileName='" & strFileNameCurrent & "']/lastModified")

        While xpiOrg.MoveNext()
            dtLastModifiedOrg = Convert.ToDateTime(xpiOrg.Current.Value)
        End While

        If xpiOrg.Count = 0 Th开发者_如何转开发en
            sbNewFiles.Append(strFileNameCurrent).Append("<br>")
        Else
            If dtLastModifiedCurrent > dtLastModifiedOrg Then
                sbModifiedFiles.Append(strFileNameCurrent).Append("<br>")
            End If
        End If

    End While