Fastest Method to (read, remove, write) to a Text File - vb.net

Question

I coded a simple program that reads from a Textfile Line by Line and If the current readed Line has alphabetics (a-z A-Z) it will write that Line into an other txt file.
If the current readed line doesn't have alphabetics it wont write that line into a new text file.
I created this for the purpose that I have members registering at my website and some of them are using only numbers as Username. I will filter them out and only save the alphabetic Names. (Focus on this Project please I know i could just use php stuff)
That works great already but it takes a while to read line by line and write into the other text file (Write speed 150kb in 1 Minute - Its not my drive I have a fast ssd).
So I wonder if there is a faster way. I could "readalllines" first but on large files it just freezes my program so I don't know if that works too (I want to focus on large +1gb files)
This is my code so far:
If System.IO.File.Exists(FILE_NAME) = True Then
Dim objReader As New System.IO.StreamReader(FILE_NAME)
Do While objReader.Peek() <> -1
Dim myFile As New FileInfo(output)
Dim sizeInBytes As Long = myFile.Length
If sizeInBytes > splitvalue Then
outcount += 1
output = outputold + outcount.ToString + ".txt"
File.Create(output).Dispose()
End If
count += 1
TextLine = objReader.ReadLine() & vbNewLine
Console.WriteLine(TextLine)
If CheckForAlphaCharacters(TextLine) Then
File.AppendAllText(output, TextLine)
Else
found += 1
Label2.Text = "Removed: " + found.ToString
TextBox1.Text = TextLine
End If
Label1.Text = "Checked: " + count.ToString
Loop
MessageBox.Show("Finish!")
End If

Answer 1

First of all, as hinted by #Sean Skelly updating UI controls - repeatedly - is an expensive operation.
But your bigger problem is File.AppendAllText:
If CheckForAlphaCharacters(TextLine) Then
File.AppendAllText(output, TextLine)
Else
found += 1
Label2.Text = "Removed: " + found.ToString
TextBox1.Text = TextLine
End If
AppendAllText(String, String)
Opens a file, appends the specified string to the file, and then
closes the file. If the file does not exist, this method creates a
file, writes the specified string to the file, then closes the file.
Source
You are repeatedly opening and closing a file, causing overhead. AppendAllText is a convenience method since it performs several operations in one single call but you can now see why it's not performing well in a big loop.
The fix is easy. Open the file once when you start your loop and close it at the end. Make sure that you always close the file properly even when an exception occurs. For that, you can either invoke the Close in a Finally block, or use a context manager, that is keep your file write operations within a Using block.
And you could remove the print to console as well. Display management has a cost too. Or you could print status updates every 10K lines or so.
When you've done all that, you should notice improved performance.

Answer 2

My Final Code - It works a lot faster now (500mbs in 1 minute)
Using sw As StreamWriter = File.CreateText(output)
For Each oneLine As String In File.ReadLines(FILE_NAME)
Try
If changeme = True Then
changeme = False
GoTo Again2
End If
If oneLine.Contains(":") Then
Dim TestString = oneLine.Substring(0, oneLine.IndexOf(":")).Trim()
Dim TestString2 = oneLine.Substring(oneLine.IndexOf(":")).Trim()
If CheckForAlphaCharacters(TestString) = False And CheckForAlphaCharacters(TestString2) = False Then
sw.WriteLine(oneLine)
Else
found += 1
End If
ElseIf oneLine.Contains(";") Or oneLine.Contains("|") Or oneLine.Contains(" ") Then
Dim oneLineReplac As String = oneLine.Replace(" ", ":")
Dim oneLineReplace As String = oneLineReplac.Replace("|", ":")
Dim oneLineReplaced As String = oneLineReplace.Replace(";", ":")
If oneLineReplaced.Contains(":") Then
Dim TestString3 = oneLineReplaced.Substring(0, oneLineReplaced.IndexOf(":")).Trim()
Dim TestString4 = oneLineReplaced.Substring(oneLineReplaced.IndexOf(":")).Trim()
If CheckForAlphaCharacters(TestString3) = False And CheckForAlphaCharacters(TestString4) = False Then
sw.WriteLine(oneLineReplaced)
Else
found += 1
End If
Else
errors += 1
textstring = oneLine
End If
Else
errors += 1
textstring = oneLine
End If
count += 1
Catch
errors += 1
textstring = oneLine
End Try
Next
End Using