ITextSharp - Different performance between file reading and TIFF page reading

Sub SurroundingSub3(FullName As String, Name As String)
    Dim Document As New iTextSharp.text.Document()
 
    Dim FirstPage As iTextSharp.text.Image = iTextSharp.text.Image.GetInstance(FullName)
    Dim MyX As Double = FirstPage.ScaledWidth / 100 * 24
    Dim MyY As Double = FirstPage.ScaledHeight / 100 * 24
    FirstPage.ScaleToFit(MyX, MyY)
    FirstPage.SetAbsolutePosition(0, 0) 
    Dim FirstPageSize As New iTextSharp.text.Rectangle(MyX, MyY) 
    Document.SetPageSize(FirstPageSize)
 
    Dim pdfFILE As String = Application.StartupPath & "" & Name & ".pdf"
    Dim DocumentWriter As iTextSharp.text.pdf.PdfWriter = iTextSharp.text.pdf.PdfWriter.GetInstance(Document, New IO.FileStream(pdfFILE, IO.FileMode.Create)) 'Inizializza il PDF
    Document.Open() 
 
    Dim myImage As iTextSharp.text.Image = iTextSharp.text.Image.GetInstance(FullName)
    myImage.ScaleToFit(MyX, MyY)
    myImage.SetAbsolutePosition(0, 0)
    Dim PageSize As New iTextSharp.text.Rectangle(MyX, MyY) 
    Document.SetPageSize(PageSize)
    Document.Add(myImage)
    Document.NewPage()
 
    Document.Close()
End Sub

Sub SurroundingSub(FullName As String, Name As String)
    Dim document As iTextSharp.text.Document = New iTextSharp.text.Document(iTextSharp.text.PageSize.A4, 0, 0, 0, 0)
    Dim writer As iTextSharp.text.pdf.PdfWriter = iTextSharp.text.pdf.PdfWriter.GetInstance(document, New System.IO.FileStream(Application.StartupPath & "" & Name & ".pdf", System.IO.FileMode.Create))
    Dim bm As System.Drawing.Bitmap = New System.Drawing.Bitmap(FullName)
    Dim total As Integer = bm.GetFrameCount(System.Drawing.Imaging.FrameDimension.Page)
    document.Open()
    Dim cb As iTextSharp.text.pdf.PdfContentByte = writer.DirectContent
 
    For k As Integer = 0 To total - 1
        bm.SelectActiveFrame(System.Drawing.Imaging.FrameDimension.Page, k)
        Dim img As iTextSharp.text.Image = iTextSharp.text.Image.GetInstance(bm, ImageFormat.Tiff)
        img.SetAbsolutePosition(0, 0)
        img.ScaleAbsoluteHeight(document.PageSize.Height)
        img.ScaleAbsoluteWidth(document.PageSize.Width)
        cb.AddImage(img)
        document.NewPage()
    Next
    document.Close()
End Sub