XPath XPathNavigator.Select（）查询在大循环中表现不佳

Question

我有一个包含500,000条记录的XML文件，其结构如下：

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

要选择特定记录，我正在使用带有查询的XPathNavigator：

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

xpiOrg是一个XPathNodeIterator。我的目标是获取'lastModified'值，其中'fileName'等于变量strFileNameCurrent，它本身是从循环中的XML文件（500,000条记录）填充的。有没有办法加快XPath查询？有没有更快捷的方法从另一个XML集中获取匹配记录？

我的整个代码是这样的：

    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 Then
            sbNewFiles.Append(strFileNameCurrent).Append("<br>")
        Else
            If dtLastModifiedCurrent > dtLastModifiedOrg Then
                sbModifiedFiles.Append(strFileNameCurrent).Append("<br>")
            End If
        End If

    End While