How to read data from richtext box and store it in array

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VB

I have to import the .txt file and segragate the nibbles(3 nibbles) and store the nibbles in an array for further processing.
i have written a code to import the .txt file to richtextbox and separate 3 nibbles and store it in array.

the below code which i have attached is working fine for small .txt file(< 100KB) but if file size is more for ex.: 2MB it is taking more time( 30 minutes ) to copy the nibbles to array.

kindly help me out to sort this problem.

the usual .txt file will be 2MB.

thanks in advance.

What I have tried:

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click

        chk = 1


        Try
            Dim TextBox1 As String : TextBox1 = "D:\temp.txt"

            Dim dialog As New OpenFileDialog()

            If DialogResult.OK = dialog.ShowDialog Then
                TextBox1 = dialog.FileName
            End If


            Dim lines() As String = System.IO.File.ReadAllLines(TextBox1)

            For Each line As String In lines
                RichTextBox1.AppendText(line)
            Next

     
            Dim dat(RichTextBox1.TextLength) As String

            For i As Integer = 0 To RichTextBox1.TextLength - 1

                dat(i) = (RichTextBox1.Text(i) & RichTextBox1.Text(i + 1) & 
                          RichTextBox1.Text(i + 2))
                i += 2

            Next
            MessageBox.Show("Done")

        Catch ex As Exception
            MessageBox.Show(ex.Message, Nothing, MessageBoxButtons.OK, 
                               MessageBoxIcon.Error)
        End Try

    End Sub