Looping over checkboxes with VBA in Excel very slow

I have an Excel Sheet with about 4500 checkboxes (I know, it sounds stupid, but it is for a customer, please do not ask...). Just wrote the VBA Sub below to uncheck all the boxes together. So far it works, but it is terribly slow, it takes more than 5 minutes until all boces are unchecked and while the Sub is running, the whole Excel Applikation grays out freezes. I know, 4500 Checkboxes is quiet a lot, but I wonder that it is really enough to bring Excel in such a trouble....Has anyone an idea?

Best Michael

Sub DeselectAll()
   Application.EnableCancelKey = False
   Application.Calculation = xlCalculationManual
   Application.ScreenUpdating = False
   Application.EnableEvents = False
   Dim wksA As Worksheet
   Dim intRow As Integer

   Set wksA = Worksheets("Companies")
   For intRow = 1 To 4513
      wksA.CheckBoxes("Checkbox_" & intRow).Value = False
   Next
 End Sub