Hi

Dave's code is using the Used Range so it will not be going beyond
column DA if that is the last column.
Maybe you have some formulae with volatile functions, which will be
causing a lot of re-calculation.
You can turn off calculation (and screen updating) at the beginning, and
back on at the end. That will probably speed things up.

Private Sub Worksheet_Activate()
   Dim myCell As Range
   Application.ScreenUpdating = False
   Application.Calculation = xlCalculationManual
   With ActiveSheet
       For Each myCell In .UsedRange.Rows(2).Cells
           myCell.EntireColumn.Hidden = CBool(myCell.Value = "")
       Next myCell
   End With
   Application.Calculation = xlCalculationAutomatic
   Application.ScreenUpdating = True
End Sub

Just to add to Roger's response...

If you can see the pagebreak dotted lines, then excel will slow down.
If you're in View|Page break preview mode, then excel will slow down.

So combining these with Roger's .screenupdating and .calculationmode changes:

Option Explicit
Sub testme()
   Dim myCell As Range
   Dim CalcMode As Long
   Dim ViewMode As Long

   Application.ScreenUpdating = False
   
   CalcMode = Application.Calculation
   Application.Calculation = xlCalculationManual
   
   ViewMode = ActiveWindow.View
   ActiveWindow.View = xlNormalView
   
   With ActiveSheet
       .DisplayPageBreaks = False
       For Each myCell In .UsedRange.Rows(1).Cells
           myCell.EntireColumn.Hidden = CBool(myCell.Value = "")
       Next myCell
   End With
   
   'put things back to what they were
   Application.Calculation = CalcMode
   ActiveWindow.View = ViewMode
   Application.ScreenUpdating = True
   
End Sub