Restricting the column range in a VBA to unmerge cells and input value

Hello,
I am trying to get the following VBA to only apply to the column range A:Z
This is a great macro that unmerges all merged cells and enters the value in to each, but it take a long time to run. Can anyone assist?

Sub UnMergeFill()
 
Dim cell As Range, joinedCells As Range
Application.ScreenUpdating = False
 
For Each cell In ThisWorkbook.ActiveSheet.UsedRange
    If cell.MergeCells Then
        Set joinedCells = cell.MergeArea
        cell.MergeCells = False
        joinedCells.Value = cell.Value
    End If
   
Next
  
Application.ScreenUpdating = True

End Sub