Why does the call from a sheet slow down the entire routine?

If I run this code from within the VBA IDE, e.g. press F5 on the Test routine, my connection and parsing the xml happens almost instantly.

Sub Test()

Sheet1.Range("4:20000").Clear

Dim con As New msxml2.ServerXMLHTTP60
    con.Open "GET", "mySite", False, "username", "password"
    con.setRequestHeader "Content-type", "application/x-www-form-urlencoded"
    con.send

Dim xmlDoc As msxml2.DOMDocument60
Set xmlDoc = con.responseXML

    parseXmlDoc xmlDoc

End Sub


Private Sub parseXmlDoc(xmlDoc As msxml2.DOMDocument60)

Dim r As Integer: r = 3
Dim c As Integer: c = 0

Dim oo As Object
Dim pp As Object
Dim qq As Object

On Error GoTo ErrHandle
With Sheet1
    Set oo = xmlDoc.SelectNodes("root/myNode")     
    For Each pp In oo
        r = r + 1
        For Each qq In pp.ChildNodes
            c = c + 1
            .Cells(r, c).Value2 = qq.nodeTypedValue
        Next qq
        c = 0
    Next pp
End With
ErrHandle:

End Sub

However, if I add a hyperlink and call the routine from there, the looping through the xml document takes about 5 seconds... Why is this happening and how can I stop it!?

Private Sub Worksheet_FollowHyperlink(ByVal Target As Hyperlink)
  Select Case Target.Range.Address(False, False)
    Case "A1"
        Test
  End Select
End Sub

I'm simply calling the routine from another place. I don't have any calculations on the sheet nor is there any other worksheet events. Like I say, if I run the Test routine from my IDE it works in milliseconds...