when i recieve value from weight bridge it is slow down

Hi guys in this code i want to recieve value in textbox1 but it more slow
how can i make this code high speed
example
in weight bridge if weight com 200kg then 250kg
it take three second to moving from 200 to 250
and when car go it moving from 250 to 0 it take 5 second
how i solve that slow
this is my code

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.IO.Ports;
using System.Threading;

namespace WindowsApplication14
{
    public partial class Form1 : Form
    {
        SerialPort _serialPort;

                private delegate void SetTextDeleg(string text);

        public Form1()
        {
            InitializeComponent();
        }

        private void btnStart_Click(object sender, EventArgs e)
        {
                        try
            {
                if (!_serialPort.IsOpen)
                    _serialPort.Open();

                _serialPort.Write("SI\r\n");
            }
            catch (Exception ex)
            {
                MessageBox.Show("Error opening/writing to serial port :: " + ex.Message, "Error!");
            }
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            // all of the options for a serial device
            // can be sent through the constructor of the SerialPort class
            // PortName = "COM1", Baud Rate = 19200, Parity = None,
            // Data Bits = 8, Stop Bits = One, Handshake = None
            _serialPort = new SerialPort("COM1", 9600, Parity.None, 8, StopBits.One);
            //_serialPort.Handshake = Handshake.None;
            _serialPort.DataReceived += new SerialDataReceivedEventHandler(sp_DataReceived);
            _serialPort.ReadTimeout = 2000;
            _serialPort.WriteTimeout = 500;
            _serialPort.Open();
        }

        void sp_DataReceived(object sender, SerialDataReceivedEventArgs e)
        {
            Thread.Sleep(500);
            string data = _serialPort.ReadLine();
            this.BeginInvoke(new SetTextDeleg(si_DataReceived), new object[] { data });
        }

        private void si_DataReceived(string data)
        {
            textBox1.Text = data.Trim();
        }
    }
}

please help me if possible