Not tested on a document that large, but you migh try my:

http://gregmaxey.mvps.org/List_Spelling_Errors.htm

Note tested on such a large document, but you might try my:

http://gregmaxey.mvps.org/List_Spelling_Errors.htm

If you don't want to use the code in my weblink then a simple version
would be:

Sub ListErrorsAllowDups()
Dim oErr As Word.Range
Dim oCol As Collection
Dim i As Long
Set oCol = New Collection
For Each oErr In ActiveDocument.Range.SpellingErrors
 oCol.Add oErr
Next
Documents.Add
For i = 1 To oCol.Count
 ActiveDocument.Range.InsertAfter oCol(i) & vbCr
Next i
End Sub

Sub ListErrorsNoDups()
Dim oErr As Word.Range
Dim oCol As Collection
Dim i As Long
Dim bDup As Boolean
Set oCol = New Collection
For Each oErr In ActiveDocument.Range.SpellingErrors
 bDup = False
 For i = 1 To oCol.Count
   If oCol(i) = oErr Then bDup = True
 Next i
 If Not bDup Then oCol.Add oErr
Next
Documents.Add
For i = 1 To oCol.Count
 ActiveDocument.Range.InsertAfter oCol(i) & vbCr
Next i
End Sub