Option Explicit
Private n       As Long

Private Sub Form_Load()
    Dim x       As Long
   
    Timer1.Interval = 10
   
    Timer1.Enabled = True
    For x = 3 To 10 ^ 6
        Call IsItPrime(x)
        DoEvents
    Next x
    Timer1.Enabled = False
    MsgBox n * 10 & " ms" & " KARCRACK"
    n = 0
   
    Timer1.Enabled = True
    For x = 3 To 10 ^ 6
        Call CheckPrimality(x)
        DoEvents
    Next x
    Timer1.Enabled = False
    MsgBox n * 10 & " ms" & " COBEIN"
    n = 0
   
    Timer1.Enabled = True
    For x = 3 To 10 ^ 6
        Call Check_Prime_Number(x)
        DoEvents
    Next x
    Timer1.Enabled = False
    MsgBox n * 10 & " ms" & " Psyke"
    n = 0
End Sub

Public Function Check_Prime_Number(ByVal lNumber As Long) As Boolean
    Const sPrimeDigit            As String = "1379"
    Dim sLastDigit               As String * 1
    Dim sNumber                  As String
    Dim x                        As Long
 
    If (lNumber > 2) Then
        sNumber = Str$(lNumber)
        sLastDigit = Right$(sNumber, 1)
        If InStr(sPrimeDigit, sLastDigit) > 0 Then
            For x = 3 To Sqr(lNumber) Step 2
                If (lNumber Mod x) = 0 Then Exit Function
            Next
            Check_Prime_Number = True
        End If
    End If
End Function

Private Function CheckPrimality(ByVal lNum As Long) As Boolean
    Dim i       As Long
    Dim lSqr    As Long
       
    If lNum Mod 2 = 0 Then GoTo Composite:

    lSqr = Sqr(lNum)

    i = 3
    Do Until i > lSqr
        If lNum Mod i = 0 Then GoTo Composite:
        i = i + 2
    Loop
   
Prime:
    CheckPrimality = True
    Exit Function
Composite:
    If lNum = 2 Then CheckPrimality = True
End Function

Public Function IsItPrime(ByVal lNumber As Long) As Boolean
    Dim i       As Long
 
    If (lNumber >= 2) And (lNumber And 1) Or (lNumber = 2) Then
        For i = 3 To Sqr(lNumber) Step 2
            If (lNumber Mod i) = 0 Then Exit Function
        Next i
        IsItPrime = True
    End If
End Function

Private Sub Timer1_Timer()
    n = n + 1
End Sub