Slow For Loop with string matching in very large ranges

[pooky333]

Hello! Please help someone... I am still at work and completely stumped. I am a relative beginner at VB and have put together some apparently awful code (as shown below). It is to compare two incredibly huge lists of strings (which might duplicate if I take out things like - (not sure if relevant!)), find matches and entering text next to each one. It is very slow and completes a loop about 4 times a minute (yes i have timed it). the annoying thing is, that it used to run quickly and now it hates me!!!!!

IDRange is 1D Range, but 15 000 rows long, and Restoring about 4000 long

How can i make it fast and why would it run quickly first and then run very slowly??? Please help... please!

Can email whole workbook if necessary.

Thanks, Becky

Sub UpdateRestoringNew()
 
  'Update Restoring, Ingested, Failed, Multiple Tapes and Recheck
  Dim IDRange As Range, Restoring As Range, Ingested As Range, Failed As Range, _
  Recheck As Range, MultipleTapes As Range, ResString As String, IDMatch As Range, _
  CellCounterRes As Integer
 
  Sheets("Ingested Content").Select
  Range("$D$5").Select
  Set IDRange = Range(Selection, Selection.End(xlDown))
 
  Sheets("Tracking").Select
  Range("$A$460").Select
  Set Restoring = Range(Selection, Selection.End(xlDown))
 
  For CellCounterRes = 1 To Restoring.Cells.Count
    ResString = Restoring.Cells(CellCounterRes).Value
 
    ' Restoring Update
    Set IDMatch = IDRange.Find(What:=ResString, LookAt:=xlPart, LookIn:=xlValues)
    If Not IDMatch Is Nothing Then
      Restoring.Cells(CellCounterRes).Offset(0, 1).Cells(1).Formula = "Updated"
      IDMatch.Offset(0, 1).Cells(1).Formula = "Restoring"
    End If
 
  Next CellCounterRes
 
End Sub