Speeding up a replace macro running through a long text

[macropod]

From your description, it seems to me all you need is:

Sub Duplicate_And_Replace()
Application.ScreenUpdating = False
Const sTEXT = "k=""ABCD"""
Const sREPLACETEXT = "k=""EFGH"""
With ActiveDocument.Range.Find
  .ClearFormatting
  .Replacement.ClearFormatting
  .Text = "([!^13]@)" & sTEXT & "([!^13]@^13)"
  .Replacement.Text = "\1" & sREPLACETEXT & "\2\1" & sTEXT & "\2"
  .Forward = True
  .Wrap = wdFindContinue
  .Format = False
  .MatchWildcards = True
  .Execute Replace:=wdReplaceAll
End With
Application.ScreenUpdating = True
End Sub