Does Visual Basic process nested loops like this very slowly, or is there some other issue with my code? - vb.net

Question

Basically, I'm trying to loop through every pixel of a picture and check it against every pixel of another image. The problem is that it seems to just do this very slowly (I can no longer interact with the opened window, and Debug.WriteLine works). I want to be sure this is the problem rather than there just being something wrong with my code.
monPic and crop are dimmed as bitmaps at the top of my code.
Private Sub BtnCheck_Click(sender As Object, e As EventArgs) Handles btnCheck.Click
monPic = New Bitmap("../../../../" & picNum & ".png")
crop = New Bitmap("../../../../mm.png")
For x As Integer = 0 To monPic.Width - 1
Debug.WriteLine("level 1")
For y As Integer = 0 To monPic.Height - 1
Debug.WriteLine("level 2")
If CInt(monPic.GetPixel(x, y).A) <> 0 Then
For x2 As Integer = 0 To crop.Width - 1
Debug.WriteLine("level 3")
For y2 As Integer = 0 To crop.Height - 1
Debug.WriteLine("level 4")
If monPic.GetPixel(x, y).R = crop.GetPixel(x2, y2).R And monPic.GetPixel(x, y).G = crop.GetPixel(x2, y2).G And monPic.GetPixel(x, y).B = crop.GetPixel(x2, y2).B Then matches += 1
Next y2
Next x2
End If
Next y
Next x
lblMatches.Text = CStr(matches)
End Sub

Answer 1

This works quickly. It requires
Imports System.Security.Cryptography
Convert the 2 bitmaps to Byte arrays then hash with Sha256. Compare the hash.
Adapted from https://www.codeproject.com/Articles/9299/Comparing-Images-using-GDI
Private Function Compare(bmp1 As Bitmap, bmp2 As Bitmap) As String
Dim result = "It's a match!"
If Not (bmp1.Size = bmp2.Size) Then
result = "It's not even the same size"
Else
Dim ic As New ImageConverter
Dim btImage1(0) As Byte
btImage1 = CType(ic.ConvertTo(bmp1, btImage1.GetType), Byte())
Dim btImage2(0) As Byte
btImage2 = CType(ic.ConvertTo(bmp2, btImage2.GetType), Byte())
Dim shaM As New SHA256Managed
Dim hash1 = shaM.ComputeHash(btImage1)
Dim hash2 = shaM.ComputeHash(btImage2)
Dim i As Integer = 0
Do While i < hash1.Length AndAlso i < hash2.Length AndAlso result = "It's a match!"
If hash1(i) <> hash2(i) Then
result = "The pixels don't match"
End If
i = (i + 1)
Loop
End If
Return result
End Function
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim png1 As New Bitmap(path1)
Dim png2 As New Bitmap(path2)
Dim message = Compare(png1, png2)
MessageBox.Show(message)
End Sub