The Signature + htmlBody Assignment is the problem. Do the commented) Signature assignment in this code and see if it fixes your speed issue.
Option Explicit

Sub Mailto_Click()
    Dim OutApp As Object
    Dim OutMail As Object
    Dim Signature As String
    Dim FullPath As String
    Dim ws As Worksheet
     
    Set OutApp = CreateObject("Outlook.Application")
    Set OutMail = OutApp.CreateItem(0)
    'Signature = OutMail.htmlbody 'Resolution problem
     
    Application.ScreenUpdating = False
    Sheet1.Visible = xlSheetHidden
    Sheet2.Visible = xlSheetHidden
    Sheet5.Visible = xlSheetHidden
    Ark6.Visible = xlSheetHidden
    
    With OutMail
        '.Display
        .To = Range("D10").Value
        .CC = ""
        .BCC = ""
        .Subject = "Papirer Bestilling"
        .htmlbody = "Sampletext" & Signature
     '*****save as pdf
        For Each ws In Sheets
            If ws.Visible = True Then
                FullPath = ThisWorkbook.Path & "\" & ws.Name & ".PDF"
                ws.ExportAsFixedFormat Type:=xlTypePDF, Filename:=FullPath
         '****add as attachements
                OutMail.Attachments.Add FullPath
           End If
        Next
    End With
    Set OutMail = Nothing
    Set OutApp = Nothing
    
    '********
    On Error Resume Next
    With Sheet1
        .Visible = xlSheetVisible
        With .ComboBox3
          If .Value = "Leie" Or .Value = "Leasing" Then
              Ark6.Visible = xlSheetVisible
              Sheet5.Visible = xlSheetVisible
              'Sheet2 ???
          End If
        End With
        .Activate
         '********
    End With

Application.ScreenUpdating = True
OutMail.Display
End Sub