Application Visible with more than 1 workbook

[p45cal]

the opened window still pops up and shows itself, even though i don*t do anything to the application. Do you know why it is?

No I don't. See the comment "Opening a new book does indeed over-ride this setting though and 'unhides' the application." here: https://stackoverflow.com/questions/...-anot#54578549

Doesn't turning off screen updating hide all workbooks which you open and subsequently close in the code?

I turned this on in the beginning and turned it off at the end (Still no performance increasement):

Public Sub SpeedUp(SpeedUpOn As Boolean)    With Application
        If SpeedUpOn Then
            .ScreenUpdating = False
            .Calculation = xlCalculationManual
            .EnableEvents = False
            .DisplayStatusBar = False 'in case you are not showing any messages
        Else
            .ScreenUpdating = True
            .Calculation = xlCalculationAutomatic
            .EnableEvents = True
            .DisplayStatusBar = True
        End If
    End With
End Sub

How slowly is the code running? Are we talking a second or two, or half an hour?
Screen updating will have a significant effect if there's lots of writing of data to sheets, Calculation to manual will have an effect if the code changes things which then require lots of calculations to take place, be it either long array-formulae and/or lots of them, disabling events if there are events to disable, status bar display will have little effect. So if your code doesn't cause any of these it will have little effect on performance.