Insert New Rows Between Rows

[Pendrant12]

Hey guys,
I found out that selecting multiple rows by hand and then inserting rows would result in inserting new rows between the selected rows.
The following macro shows this:

Sub Macro1()
    Range("1:1,2:2,3:3").Select
    Selection.Insert Shift:=xlDown, CopyOrigin:=xlFormatFromLeftOrAbove
End Sub

Now, what I need to do is this:

Sub test()
    'Getting the last Row
    Cells(1, 1).Select
    lastRow = Selection.End(xlDown).Row
    'Building the Selection
    For i = 1 To lastRow
        selectionString = selectionString & i & ":" & i & ","
    Next i
    'Removing the last ","
    selectionString = Mid(selectionString, 1, Len(selectionString) - 1)
    Range(selectionString).Select
    Selection.Insert Shift:=xlDown, CopyOrigin:=xlFormatFromLeftOrAbove
End Sub

This is trying to select each row individually to insert rows between the selected rows. As expected, I get the following error as it tries to select the rows:

Run-time error '1004':

Method 'Range' of object 'Global_' failed

I know there are plenty of ways to achieve the same result by creating a simple for-loop and inserting a new row after each row.
This is NOT what I want to do, since this seems to be a lot slower. (I am speaking about 200.000-300.000 rows)
Any ideas no how to get this working without a loop?