Speed up large string data parser function - vb.net

Question

I currently have a file with 1 million characters.. the file is 1 MB in size. I am trying to parse data with this old function that still works but very slow.
start0end
start1end
start2end
start3end
start4end
start5end
start6end
the code, takes about 5 painful minutes to process the whole data.
any pointers and suggestions are appreciated.
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim sFinal = ""
Dim strData = textbox.Text
Dim strFirst = "start"
Dim strSec = "end"
Dim strID As String, Pos1 As Long, Pos2 As Long, strCur As String = ""
Do While InStr(strData, strFirst) > 0
Pos1 = InStr(strData, strFirst)
strID = Mid(strData, Pos1 + Len(strFirst))
Pos2 = InStr(strID, strSec)
If Pos2 > 0 Then
strID = Microsoft.VisualBasic.Left(strID, Pos2 - 1)
End If
If strID <> strCur Then
strCur = strID
sFinal += strID & ","
End If
strData = Mid(strData, Pos1 + Len(strFirst) + 3 + Len(strID))
Loop
End Sub

Answer 1

The reason that is so slow is because you keep destroying and recreating a 1 MB string over and over. Strings are immutable, so strData = Mid(strData... creates a new string and copies the remaining of the 1 MB string data to a new strData variable over and over and over. Interestingly, even VB6 allowed for a progressive index.
I would have processed the disk file LINE BY LINE and plucked out the info as it was read (see streamreader.ReadLine) to avoid working with a 1MB string. Pretty much the same method could be used there.
' 1 MB textbox data (!?)
Dim sData As String = TextBox1.Text
' start/stop - probably fake
Dim sStart As String = "start"
Dim sStop As String = "end"
' result
Dim sbResult As New StringBuilder
' progressive index
Dim nNDX As Integer = 0
' shortcut at least as far as typing and readability
Dim MagicNumber As Integer = sStart.Length
' NEXT index of start/stop after nNDX
Dim i As Integer = 0
Dim j As Integer = 0
' loop as long as string remains
Do While (nNDX < sData.Length) AndAlso (i >= 0)
i = sData.IndexOf(sStart, nNDX) ' start index
j = sData.IndexOf(sStop, i) ' stop index
' Extract and append bracketed substring
sbResult.Append(sData.Substring(i + MagicNumber, j - (i + MagicNumber)))
' add a cute comma
sbResult.Append(",")
nNDX = j ' where we start next time
i = sData.IndexOf(sStart, nNDX)
Loop
' remove last comma
sbResult.Remove(sbResult.ToString.Length - 1, 1)
' show my work
Console.WriteLine(sbResult.ToString)
EDIT: Small mod for the ad hoc test data