C#自定义RichTextBox加载文件死慢！！

在网上抄了段代码，作用【关闭自动选择字符/生僻字显示】。
测试一下发现加载小文档没问题（秒显），但加载长文档慢得让人抓狂（如加载五笔词库，大概76000+行），要十几二十秒才显示出来，过程就像假死一样。

VS自带的RichTextBox 加载五笔词库大概3秒以内。这个问题出在哪里？？？

请大神指点优化一下。

using System;

using System.ComponentModel;

using System.IO;

using System.Runtime.InteropServices;

using System.Windows.Forms;



namespace 文件加载速度测试

{

    public partial class ExRichTextBox : RichTextBox

    {

        #region 自定义变量

        const string DLL_RICHEDIT = "msftedit.dll";

        /// <summary>MSFTEDIT.DLL（即RichEdit4.1版本）注册的类名是"RICHEDIT50W"</summary>

        const string WC_RICHEDITW = "RICHEDIT50W";



        /// <summary>用于表示指针或句柄的平台特定类型。</summary>

        private IntPtr moduleHandle;

        /// <summary>获取或设置一个值，尝试加载</summary>

        private bool attemptedLoad;

        /// <summary>用于判断鼠标是否按下</summary>

        private bool _Selecting = false;

        /// <summary>记录鼠标按下时的位置</summary>

        private int _StartPosition = 0;



        /// <summary> 加载库 </summary>

        /// <param name="libname">库名</param>

        /// <returns></returns>

        [DllImport("Kernel32.dll")]

        internal static extern IntPtr LoadLibrary(string libname);

        #endregion



        public ExRichTextBox()

        {

            InitializeComponent();



            moduleHandle = IntPtr.Zero;

            attemptedLoad = false;

            AutoWordSelection = false;//关闭自动选字词

            Font = new System.Drawing.Font(

                "宋体", 12.0F, System.Drawing.FontStyle.Regular,

                System.Drawing.GraphicsUnit.Point, 0);

            /* 文本中出现‘西方语言’用西方字体显示，出现‘亚洲语言’则用亚洲字体显示 */

            this.LanguageOption = RichTextBoxLanguageOptions.DualFont;

            ShowSelectionMargin = true;

            ImeMode = ImeMode.On;

        }



        public ExRichTextBox(IContainer container)

        {

            container.Add(this);



            InitializeComponent();



            moduleHandle = IntPtr.Zero;

            attemptedLoad = false;

            AutoWordSelection = false;//关闭自动选字词

            Font = new System.Drawing.Font(

                "宋体", 12.0F, System.Drawing.FontStyle.Regular,

                System.Drawing.GraphicsUnit.Point, 0);

            ShowSelectionMargin = true;

            ImeMode = ImeMode.On;

        }



        #region 重写控件信息

        /// <summary>重写控件的信息</summary>

        protected override CreateParams CreateParams

        {

            get

            {

                CreateParams cp = base.CreateParams;

                AttemptToLoadNewRichEdit();

                if (moduleHandle != IntPtr.Zero)

                {

                    cp.ClassName = WC_RICHEDITW;

                }

                return cp;

            }

        }



        /// <summary>尝试加载新的丰富的编辑(RichEdit)信息</summary>

        void AttemptToLoadNewRichEdit()

        {

            // 检索动态库链接

            if (!attemptedLoad)

            {

                attemptedLoad = true;

                string strFile = Path.Combine(Environment.SystemDirectory, DLL_RICHEDIT);

                moduleHandle = LoadLibrary(strFile);

            }

        }

        #endregion



        #region 重写鼠标事件

        /// <summary>重写鼠标按下事件</summary>

        /// <param name="e">鼠标点击事件</param>

        protected override void OnMouseDown(MouseEventArgs e)

        {

            _Selecting = true;

            _StartPosition = base.GetCharIndexFromPosition(new System.Drawing.Point(e.X, e.Y));

            base.OnMouseDown(e);

        }



        /// <summary>重写鼠标释放事件</summary>

        /// <param name="e">鼠标点击事件</param>

        protected override void OnMouseUp(MouseEventArgs e)

        {

            AutoWordSelection = false;/* 关闭自动选字词 此项必须 */

            _Selecting = false;

            base.OnMouseUp(e);

            if (_Selecting)

            {

                int charPosition = base.GetCharIndexFromPosition(new System.Drawing.Point(e.X, e.Y));

                int length = 0;

                if (charPosition > _StartPosition)

                {

                    length = charPosition - _StartPosition;

                    Select(_StartPosition, length);

                }

                else if (charPosition < _StartPosition)

                {

                    length = _StartPosition - charPosition;

                    Select(charPosition, length);

                }

            }

        }

        #endregion

    }

}

string str2 = @"D:\Documents\五笔词库20160202.txt";

        private void button1_Click(object sender, EventArgs e)

        {

            exRichTextBox1.Text = File.ReadAllText(str2, Encoding.Default);

            //exRichTextBox1.Lines = File.ReadAllLines(str2, Encoding.Default);

        }



        private void button2_Click(object sender, EventArgs e)

        {

            exRichTextBox1.Clear();

        }