更快的写XML的方法

我写的第一个程序中写XML的代码执行速度有些问题，改了一下，现在有所改善。
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Diagnostics;
using System.Linq;
using System.ServiceProcess;
using System.Text;
using System.Data.SqlClient;
using System.Xml;
using System.IO;
using System.Collections;
using System.Threading;
namespace StockUpService
{
public partial class StockUpService : ServiceBase
{
public StockUpService()
{
InitializeComponent();
if (!System.Diagnostics.EventLog.SourceExists("MySource"))
{
System.Diagnostics.EventLog.CreateEventSource(
"MySource", "MyNewLog");
}
eventLog1.Source = "MySource";
eventLog1.Log = "MyNewLog";
}
protected override void OnStart(string[] args)
{
System.Timers.Timer t = new System.Timers.Timer(20000); //实例化Timer类，设置间隔时间为20000毫秒|20秒执行一次；
t.Elapsed += new System.Timers.ElapsedEventHandler(runservice); //到达时间的时候执行事件；
t.AutoReset = true; //设置是执行一次（false）还是一直执行(true)；
t.Enabled = true; //是否执行System.Timers.Timer.Elapsed事件；
}
public void runservice(object source, System.Timers.ElapsedEventArgs e)
{
string t;
t = System.DateTime.Now.Hour.ToString()+System.DateTime.Now.Minute.ToString();
if (t == "933")
{
readXml();
}
}
public static void readXml()
{
string classId="";
string filialeId;
string startTime;
string endTime;
string companyType;
string xmlChildNodes = "";