If all of the headings/sections were already outlined and you didn't need to mark/flag which parts of the document were needed to become part of the index then yes, you could do it very quickly.
If the document had no sections and you had to go through each of the 125 pages marking where you wanted headings/sections, then you would be at it substantially longer.
It sounds like you're confusing an index and a table of contents. Indexing a 125 page document properly could well be several days work. It depends on how detailed the index needs to be.
If every word in the document was indexed then could you 'select all' the document and 'insert index'? In this instance the index would not be detailed and therefore it could be done very quickly!!!