What causes a macro to run slower at certain times?

I couldn't find anything that really answered this question specifically. When I run macros usually they're fast, but occasionally will run very slow. An example macro I have is pasted below. It's just a simple macro that pastes months. If I am pasting on top of a cell that contains a month it pastes the rest of the months that follow.

Most of the time this macro runs very fast, but occasionally it takes about 1-2 seconds to run, and it is a very simple macro! It is not macro specific, as all my macros will be slow. I''m using Windows 7 with Excel 2010. I'm only using 23% of my CPU and 4.5GB of my 16GB of RAM, so I don't think it's a hardware thing.

Is there some kind of condition that occurs that will make excel run macros slower? Any help is appreciated.

Sub months()
    Dim monthArray(12) As String
    Dim index As Integer
    
    'I cannot believe there isn't a better way to do this
    monthArray(0) = "January"
    monthArray(1) = "February"
    monthArray(2) = "March"
    monthArray(3) = "April"
    monthArray(4) = "May"
    monthArray(5) = "June"
    monthArray(6) = "July"
    monthArray(7) = "August"
    monthArray(8) = "September"
    monthArray(9) = "October"
    monthArray(10) = "November"
    monthArray(11) = "December"

    If Selection.Count = 1 Then

        For counter = 0 To 11
            If LCase(Selection) = LCase(monthArray(counter)) Then
                index = counter
            End If
        
        Next counter
        
        monthsLeft = 11 - index
        
        For monthCounter = 0 To monthsLeft
            ActiveCell.Offset(, monthCounter).Range("A1") = monthArray(index + monthCounter)
        Next monthCounter
        
        If LCase(Selection.Value) = "december" Then
            For monthCounter = 1 To 12
                ActiveCell.Offset(, monthCounter).Range("A1") = monthArray(monthCounter - 1)
            Next monthCounter
        End If
            
    End If
    
End Sub