Why VB.NET simple app takes forever to load several lines - vb.net

Question

I have a super simple script that I am using to separate a long list of phone numbers we've gathered from donors over the years into separate area code files.
Obviously, when you have almost 1 million lines it's going to take a while - BUT - if I put in 1,000 it takes less than a second. Once I put in 1 million it takes 10 seconds to do only 5 lines. How could this be?
Imports System.IO
Public Class Form1
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
System.Windows.Forms.Control.CheckForIllegalCrossThreadCalls = False
BackgroundWorker1.RunWorkerAsync()
End Sub
Private Sub BackgroundWorker1_DoWork(ByVal sender As System.Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker1.DoWork
System.Windows.Forms.Control.CheckForIllegalCrossThreadCalls = False
Dim lines As String
lines = RichTextBox1.Lines.Count
Dim looper As String = 0
Dim path As String = "c:\acs\"
MsgBox("I have " + lines + " lines to do.")
If (Not System.IO.Directory.Exists(path)) Then
System.IO.Directory.CreateDirectory(path)
End If
Dim i As String = 0
For loops As Integer = 0 To RichTextBox1.Lines.Count - 1
Dim ac As String = RichTextBox1.Lines(looper).ToString
ac = ac.Substring(0, 3)
Dim strFile As String = path + ac + ".txt"
Dim sw As StreamWriter
sw = File.AppendText(strFile)
sw.WriteLine(RichTextBox1.Lines(looper))
sw.Close()
Label1.Text = String.Format("Processing item {0} of {1}", looper, lines)
looper = looper + 1
Next
MsgBox("done now")
End Sub
End Class

Answer 1

Each time you use the RichTextBox.Lines properties, VB.Net will need to split the content by CR+LF pair. Thus your For loops As Integer = - To RichTextBox1.Lines.Count-1 is really a performance hit.
Try to use:
For Each vsLine As String in RichTextBox1.Lines
instead. It will be a lot faster. Alternatively, if you must use For loop, then get it once:
Dim vasLines() As String = RichTextBox1.Lines
For viLines As Integer = 0 to UBound(vasLines.Count)
'....
Next
instead.

Answer 2

First, you're performing UI updates in your For loop. That will take time.
You're updating the UI in a thread that is not the main thread which might impact performance. You should not use the CheckForIllegalCrossThreadCalls method. You should update the UI properly using the ReportProgress method of the BackgroundWorker.
You are opening and closing a file for each iteration of the loop. That will take time as well.
I think a better method would be to add the data to a Dictionary(Of String, List(Of String)), with the area code as the key and the list would hold all the number for that area code. Once the Dictionary is filled, loop through the keys and write the numbers out.