Excel Slow performance

[SamT]

100% Arrays:

Sub M_snb()
   sn = Sheets("Data").Cells(1).CurrentRegion
   sp = Sheets("Mapping").UsedRange
   
   For j = 2 To UBound(sn)
       For jj = 2 To UBound(sp)
          If sn(j, 3) = sp(jj, 1) Then
             sn(j, 17) = sp(jj, 2)
             sn(j, 18) = sp(jj, 4)
             sn(j, 19) = sp(jj, 5)
             Exit For
          End If
      Next
   
       For jj = 2 To UBound(sp)
          If sn(j, 3) = sp(jj, 16) Then
             sn(j, 20) = sp(jj, 20)
             sn(j, 21) = sp(jj, 21)
             Exit For
          End If
      Next
      
      w_00 = sn(j, 2) - Weekday(sn(j, 2))
      For jj = 22 To 28
        If jj < 27 Then sn(j, jj - 10) = Format(Choose(jj - 21, sn(j, 2), sn(j, 2), sn(j, 2), _
          w_00 + 1, w_00 + 7), Choose(jj - 21, "dddd", "'mmm-yy", "\Wk ww", _
          "\WB dd-mm-yyyy", "\WE dd-mm-yyyy"))

        sn(j, jj) = Choose(jj - 21, sn(j, 4) * sn(j, 5), sn(j, 4) * sn(j, 6), sn(j, 4) _
         * sn(j, 7), sn(j, 4) * sn(j, 8), sn(j, 9) / 600, sn(j, 10) / 60, sn(j, 11) / 60)
      Next
   Next
   
   Sheets("Data").Cells(1).CurrentRegion.Offset(19) = sn
End Sub

Sheet "mapping" columns I:N are redundant.
You don't even need screenupdating =false.

Does this do the same thing

    sn = Sheets("Data").Cells(1).CurrentRegion 
    sp = Sheets("Mapping").UsedRange 
     
    For j = 2 To UBound(sn) 
        For jj = 2 To UBound(sp) 
            If sn(j, 3) = sp(jj, 1) Then 
                sn(j, 17) = sp(jj, 2) 
                sn(j, 18) = sp(jj, 4) 
                sn(j, 19) = sp(jj, 5) 
If jj > 21 And jj < 29 Then GoTo jj2228
                Exit For 
             ElseIf sn(j, 3) = sp(jj, 16) Then 
                sn(j, 20) = sp(jj, 20) 
                sn(j, 21) = sp(jj, 21) 
If jj > 21 And jj < 29 Then GoTo jj2228
                Exit For 
            End If 

jj2228:        
If  jj > 21 And jj < 29 Then
        w_00 = sn(j, 2) - Weekday(sn(j, 2)) 

             If jj < 27 Then 
sn(j, jj - 10) = Format(Choose(jj - 21, sn(j, 2), sn(j, 2), sn(j, 2), w_00 + 1, w_00 + 7), _
Choose(jj - 21, "dddd", "'mmm-yy", "\Wk ww", "\WB dd-mm-yyyy", "\WE dd-mm-yyyy")) 
End If
            
sn(j, jj) = Choose(jj - 21, sn(j, 4) * sn(j, 5), sn(j, 4) * sn(j, 6), sn(j, 4) * sn(j, 7), sn(j, 4) _
* sn(j, 8), sn(j, 9) / 600, sn(j, 10) / 60, sn(j, 11) / 60) 
End If
        Next 
    Next 
     
    Sheets("Data").Cells(1).CurrentRegion.Offset(19) = sn 
End Sub