Many processes need to write large volumes of data in Excel.  The typical method is to loop through each cell and perform the action. 

   Dim CellsDown As Long
   CellsAcross As Long

   Dim CurrRow As Long
   CurrCol As Long
   Dim CurrVal As Long

   '  This can be replaced with the selected range and is just used to illustrate this example.
   CellsDown = 1000
   CellsAcross = 36

   '   Loop through cells and insert values
   CurrVal = 1
   Application.ScreenUpdating = False
   For CurrRow = 1 To CellsDown
       For CurrCol = 1 To CellsAcross
           Range("A1").Offset(CurrRow - 1, CurrCol - 1).Value = CurrVal
           CurrVal = CurrVal + 1
       Next CurrCol
   Next CurrRow

Rather than writing the values out cell by cell, it is quicker to store the value in an array and write the array to a range of cells at one time.

   Dim CellsDown As Long
   CellsAcross As Long

   Dim CurrRow As Long
   CurrCol As Long
   Dim CurrVal As Long

   Dim TempArray() As Double

   '  This can be replaced with the selected range and is just used to illustrate this example.
   CellsDown = 1000
   CellsAcross = 36

   '   Update the array
   ReDim TempArray(1 To CellsDown, 1 To CellsAcross)
   Set TheRange = Range(Cells(1, 1), Cells(CellsDown, CellsAcross))

   '   Fill the temporary array
   CurrVal = 0
   Application.ScreenUpdating = False
   For i = 1 To CellsDown
       For j = 1 To CellsAcross
           TempArray(i, j) = CurrVal
           CurrVal = CurrVal + 1
       Next j
   Next i

   '   Transfer temporary array to worksheet
   TheRange.Value = TempArray

This same method can be used when altering data.  By changing the following line

            TempArray(i, j) = CurrVal

To this

            TempArray(i, j) = TheRange(i, j) * 3

By using TheRange(i, j), the existing value can be altered

The process of writing values cell by cell took 3.16 seconds.  Using the array method, it took .08 seconds, nearly 40 times faster