Tableau trop lourd

Option Explicit
Sub MFC_sous_totaux()
Dim c As Range
For Each c In Sheets("2013").Range("A2", Range("A65536").End(xlUp))
    If IsDate(c) Then
        With c.Resize(, 3)
            .Interior.ColorIndex = 36
            .Font.Bold = True
            .Font.Size = 11
        End With
    
        With c.Resize(, 3).Borders(xlEdgeTop)
            .Weight = xlMedium
        End With
    
        With c.Resize(, 3).Borders(xlEdgeBottom)
            .Weight = xlMedium
        End With
    End If
Next
End Sub

Sub MFC_sous_totauxREGIMEPHASEI()
Dim c As Range
For Each c In Sheets(1).Range("A2", Range("A65536").End(xlUp))
    If IsDate(c) Then
        With c.Resize(, 3)
        .Borders(8).Weight = xlMedium
        .Borders(9).Weight = xlMedium
        .Interior.ColorIndex = 36
        .Font.Bold = True
        .Font.Size = 11
        End With
    End If
Next
End Sub

Sub MFC_sous_totauxREGIMEPHASEII()
Dim c As Range
For Each c In Sheets(1).Range("A2", Range("A65536").End(xlUp))
    If IsDate(c) Then
        With c.Resize(, 3)
        .BorderAround 1, -4138
        .Interior.ColorIndex = 36
        .Font.Bold = True
        .Font.Size = 11
        End With
    End If
Next
End Sub

Sub tentative()
Dim ThF As Worksheet, F As Worksheet
Set ThF = ActiveSheet
Application.ScreenUpdating = False
        For Each F In Worksheets
       F.Activate
      ActiveSheet.UsedRange
Next F
ThF.Activate
End Sub